[OpenMP] RISCV64 port

This is a port of libomp for the RISC-V 64-bit Linux target.

We have tested this port on a HiFive Unleashed development board
using a downstream LLVM that has support for the missing bits in
upstream. As of now, all tests are passing, including OMPT.

Patch by Ferran Pallarès!

Differential Revision: https://reviews.llvm.org/D59880

llvm-svn: 367021
diff --git a/openmp/runtime/tools/lib/Uname.pm b/openmp/runtime/tools/lib/Uname.pm
index 4a5c332..a14cb3a 100644
--- a/openmp/runtime/tools/lib/Uname.pm
+++ b/openmp/runtime/tools/lib/Uname.pm
@@ -156,6 +156,8 @@
         $values{ hardware_platform } = "mips64";
     } elsif ( $values{ machine } =~ m{\Amips\z} ) {
         $values{ hardware_platform } = "mips";
+    } elsif ( $values{ machine } =~ m{\Ariscv64\z} ) {
+        $values{ hardware_platform } = "riscv64";
     } else {
         die "Unsupported machine (\"$values{ machine }\") returned by POSIX::uname(); stopped";
     }; # if