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.

1 files changed, 69 insertions, 0 deletions

diff --git a/riscV/CBuiltins.ml b/riscV/CBuiltins.ml
new file mode 100644
index 00000000..385f9d13
--- /dev/null
+++ b/riscV/CBuiltins.ml
@@ -0,0 +1,69 @@
+(* *********************************************************************)
+(*                                                                     *)
+(*              The Compcert verified compiler                         *)
+(*                                                                     *)
+(*          Xavier Leroy, INRIA Paris-Rocquencourt                     *)
+(*                                                                     *)
+(*  Copyright Institut National de Recherche en Informatique et en     *)
+(*  Automatique.  All rights reserved.  This file is distributed       *)
+(*  under the terms of the GNU General Public License as published by  *)
+(*  the Free Software Foundation, either version 2 of the License, or  *)
+(*  (at your option) any later version.  This file is also distributed *)
+(*  under the terms of the INRIA Non-Commercial License Agreement.     *)
+(*                                                                     *)
+(* *********************************************************************)
+
+(* Processor-dependent builtin C functions *)
+
+open C
+
+let builtins = {
+  Builtins.typedefs = [
+    "__builtin_va_list", TPtr(TVoid [], [])
+  ];
+  Builtins.functions = [
+    (* Synchronization *)
+    "__builtin_fence",
+      (TVoid [], [], false);
+    (* Integer arithmetic *)
+    "__builtin_bswap",
+      (TInt(IUInt, []), [TInt(IUInt, [])], false);
+    "__builtin_bswap64",
+      (TInt(IULongLong, []), [TInt(IULongLong, [])], false);
+    "__builtin_bswap32",
+      (TInt(IUInt, []), [TInt(IUInt, [])], false);
+    "__builtin_bswap16",
+      (TInt(IUShort, []), [TInt(IUShort, [])], false);
+    (* Float arithmetic *)
+    "__builtin_fmadd",
+      (TFloat(FDouble, []),
+       [TFloat(FDouble, []); TFloat(FDouble, []); TFloat(FDouble, [])],
+       false);
+    "__builtin_fmsub",
+      (TFloat(FDouble, []),
+       [TFloat(FDouble, []); TFloat(FDouble, []); TFloat(FDouble, [])],
+       false);
+    "__builtin_fnmadd",
+      (TFloat(FDouble, []),
+       [TFloat(FDouble, []); TFloat(FDouble, []); TFloat(FDouble, [])],
+       false);
+    "__builtin_fnmsub",
+      (TFloat(FDouble, []),
+       [TFloat(FDouble, []); TFloat(FDouble, []); TFloat(FDouble, [])],
+       false);
+    "__builtin_fsqrt",
+      (TFloat(FDouble, []), [TFloat(FDouble, [])], false);
+    "__builtin_fmax",
+      (TFloat(FDouble, []), [TFloat(FDouble, []); TFloat(FDouble, [])], false);
+    "__builtin_fmin",
+      (TFloat(FDouble, []), [TFloat(FDouble, []); TFloat(FDouble, [])], false);
+  ]
+}
+
+let va_list_type = TPtr(TVoid [], [])  (* to check! *)
+let size_va_list = if Archi.ptr64 then 8 else 4
+let va_list_scalar = true
+
+(* Expand memory references inside extended asm statements.  Used in C2C. *)
+
+let asm_mem_argument arg = Printf.sprintf "0(%s)" arg