The LLVM test suite has its own way of detecting the system
architecture. This adds support in that file for detecting RISC-V.
This will eventually cause the ARCH variable to be populated.
We use ARCH="riscv64" to identify 64-bit RISC-V, and ARCH="riscv32" to identify 32-bit
RISC-V, so that attempting to detect "riscv" in the ARCH variable will match any version of RISC-V.