[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