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

[Kito Cheng via cfe-commits]

https://github.com/kito-cheng approved this pull request.

Checked with `Generic_GCC::GCCInstallationDetector::init` to make sure clang will use that to search gcc toolchain, so LGTM.

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