[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

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@367021 91177308-0d34-0410-b5e6-96231b3b80d8