[Driver][RISCV] Add RedHat Linux RISC-V triple
Summary: Adds the RedHat Linux triple to the list of 64-bit RISC-V triples.
Without this the gcc libraries wouldn't be found by clang on a redhat/fedora
system, as the search list included /usr/lib/gcc/riscv64-redhat-linux-gnu
but the correct path didn't include the -gnu suffix.
Reviewers: lenary, asb, dlj
Reviewed By: lenary
Differential Revision: https://reviews.llvm.org/D74399