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, 3 insertions, 3 deletions

diff --git a/runtime/c/i64_dtou.c b/runtime/c/i64_dtou.c
index c5f9df4d..5f4901b2 100644
--- a/runtime/c/i64_dtou.c
+++ b/runtime/c/i64_dtou.c
@@ -38,7 +38,7 @@
 
 /* Conversion float64 -> unsigned int64 */
 
-unsigned long long __i64_dtou(double d)
+unsigned long long i64_dtou(double d)
 {
   /* Extract bits of d's representation */
   union { double d; unsigned long long i; } buf;
@@ -62,8 +62,8 @@ unsigned long long __i64_dtou(double d)
     (buf.i & ~0xFFF0000000000000LL) | 0x0010000000000000LL;
   /* Shift it appropriately */
   if (e >= 0)
-    return __i64_shl(m, e);
+    return i64_shl(m, e);
   else
-    return __i64_shr(m, -e);
+    return i64_shr(m, -e);
 
 }