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

[via cfe-commits]

mihailo-stojanovic wrote:

I believe that more information can be found in the [commit](https://github.com/llvm/llvm-project/commit/45ba2392d7e00aedd4d9fb04070dbae8a7fbbeeb) which added the `RISCVToolChain::hasGCCToolchain` method. As far as I could gather, before this was merged, the only option for baremetal compilation was to provide an existing GCC installation, as `toolchains::RISCVToolChain` expects one to be present.

My patch just makes sure that `--gcc-install-dir` works as intended with RISCV baremetal triples, as the option is currently useless, and I believe the `-gcc-toolchain` option is deprecated.

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