RISC-V port and assorted changes

This commits adds code generation for the RISC-V architecture, both in 32- and 64-bit modes.

The generated code was lightly tested using the simulator and cross-binutils from https://riscv.org/software-tools/

This port required the following additional changes:

- Integers: More properties about shrx

- SelectOp: now provides smart constructors for mulhs and mulhu

- SelectDiv, 32-bit integer division and modulus: implement constant propagation, use the new smart constructors mulhs and mulhu.

- Runtime library: if no asm implementation is provided, run the reference C implementation through CompCert.  Since CompCert rejects the definitions of names of special functions such as __i64_shl, the reference implementation now uses "i64_" names, e.g. "i64_shl", and a renaming "i64_ -> __i64_" is performed over the generated assembly file, before assembling and building the runtime library.

- test/: add SIMU make variable to run tests through a simulator

- test/regression/alignas.c: make sure _Alignas and _Alignof are not #define'd by C headers

commit da14495c01cf4f66a928c2feff5c53f09bde837f
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Thu Apr 13 17:36:10 2017 +0200

    RISC-V port, continued

    Now working on Asmgen.

commit 36f36eb3a5abfbb8805960443d087b6a83e86005
Author: Xavier Leroy <xavier.leroy@inria.fr>
Date:   Wed Apr 12 17:26:39 2017 +0200

    RISC-V port, first steps

    This port is based on Prashanth Mundkur's experimental RV32 port and brings it up to date with CompCert, and adds 64-bit support (RV64).  Work in progress.

2 files changed, 153 insertions, 0 deletions

diff --git a/runtime/riscV/sysdeps.h b/runtime/riscV/sysdeps.h
new file mode 100644
index 00000000..b95ca1b1
--- /dev/null
+++ b/runtime/riscV/sysdeps.h
@@ -0,0 +1,63 @@
+// *****************************************************************
+//
+//               The Compcert verified compiler
+//
+//           Xavier Leroy, INRIA Paris-Rocquencourt
+//
+// Copyright (c) 2013 Institut National de Recherche en Informatique et
+//  en Automatique.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above copyright
+//       notice, this list of conditions and the following disclaimer in the
+//       documentation and/or other materials provided with the distribution.
+//     * Neither the name of the <organization> nor the
+//       names of its contributors may be used to endorse or promote products
+//       derived from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT
+// HOLDER> BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// *********************************************************************
+
+// System dependencies
+
+#define FUNCTION(f) \
+	.text; \
+        .balign 16; \
+	.globl f; \
+f:
+
+#define ENDFUNCTION(f) \
+	.type f, @function; .size f, . - f
+
+#if defined(MODEL_64)
+
+#define WORDSIZE 8
+#define lptr ld
+#define sptr sd
+
+#elif defined(MODEL_32)
+
+#define WORDSIZE 4
+#define lptr lw
+#define sptr sw
+
+#else
+
+#error "Wrong MODEL"
+
+#endif
+
diff --git a/runtime/riscV/vararg.S b/runtime/riscV/vararg.S
new file mode 100644
index 00000000..a9481077
--- /dev/null
+++ b/runtime/riscV/vararg.S
@@ -0,0 +1,90 @@
+// *****************************************************************
+//
+//               The Compcert verified compiler
+//
+//           Xavier Leroy, INRIA Paris-Rocquencourt
+//           Prashanth Mundkur, SRI International
+//
+// Copyright (c) 2013 Institut National de Recherche en Informatique et
+//  en Automatique.
+//
+//  The contributions by Prashanth Mundkur are reused and adapted
+//  under the terms of a Contributor License Agreement between
+//  SRI International and INRIA.
+//	
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above copyright
+//       notice, this list of conditions and the following disclaimer in the
+//       documentation and/or other materials provided with the distribution.
+//     * Neither the name of the <organization> nor the
+//       names of its contributors may be used to endorse or promote products
+//       derived from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT
+// HOLDER> BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// *********************************************************************
+
+// Helper functions for variadic functions <stdarg.h>.  RISC-V version.
+
+#include "sysdeps.h"	
+	
+// typedef void * va_list;
+// unsigned int __compcert_va_int32(va_list * ap);
+// unsigned long long __compcert_va_int64(va_list * ap);
+// double __compcert_va_float64(va_list * ap);
+
+FUNCTION(__compcert_va_int32)
+                                   # a0 = ap parameter
+	lptr	t5, 0(a0)	   # t5 = pointer to next argument
+	addi	t5, t5, WORDSIZE   # advance ap
+	sptr	t5, 0(a0)	   # update ap
+	lw	a0, -WORDSIZE(t5)  # load it and return it in a0
+	jr	ra
+ENDFUNCTION(__compcert_va_int32)
+
+FUNCTION(__compcert_va_int64)
+				# a0 = ap parameter
+	lptr	t5, 0(a0)	# t5 = pointer to next argument
+	addi	t5, t5, 15	# 8-align and advance by 8
+	and	t5, t5, -8
+	sptr	t5, 0(a0)	# update ap
+#ifdef MODEL_64
+	ld	a0, -8(t5)	# return it in a0
+#else
+	lw	a0, -8(t5)	# return it in [a0,a1]
+	lw	a1, -4(t5)
+#endif
+	jr	ra
+ENDFUNCTION(__compcert_va_int64)
+
+FUNCTION(__compcert_va_float64)
+				# a0 = ap parameter
+	lptr	t5, 0(a0)	# t5 = pointer to next argument
+	addi	t5, t5, 15	# 8-align and advance by 8
+	and	t5, t5, -8
+	sw	t5, 0(a0)	# update ap
+	fld	fa0, -8(t5)	# return it in fa0
+	jr	ra
+ENDFUNCTION(__compcert_va_float64)
+
+// Right now we pass structs by reference.  This is not ABI conformant.
+FUNCTION(__compcert_va_composite)
+#ifdef MODEL_64
+	j	__compcert_va_int64
+#else
+	j	__compcert_va_int32
+#endif
+ENDFUNCTION(__compcert_va_composite)