[clang] [clang] Enable --gcc-install-dir for RISCV baremetal toolchains (PR #71803)

[Fangrui Song via cfe-commits]

MaskRay wrote:

I am not familiar with RISC-V baremetal toolchains, but this toolchain gave me a feeling of "there is a lot of fragmentation in this area. However, we as a vendor upstreamed first what we did to clang/lib/Driver/ToolChains/RISCVToolchain.cpp and made it apply to all target triples without an OS (Linux/*BSD)"

https://github.com/llvm/llvm-project/pull/71803