[RISCV] Collect library directories and triples for riscv64 triple too

When setting up library and tools paths when detecting an accompanying GCC
installation only riscv32 was handled. As a consequence when targetting
riscv64 neither the linker nor libraries would be found. This adds handling
and tests for riscv64.

Differential Revision: https://reviews.llvm.org/D53392
Patch by Edward Jones.

llvm-svn: 357699