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

[Fangrui Song via cfe-commits]

MaskRay wrote:

A user has recently asked me about `RISCVToolChain` and I am puzzled by its behavior.

Why is the return value of `RISCVToolChain::hasGCCToolchain` used to choose `toolchains::RISCVToolChain` or `toolchains::BareMetal`? The logic should be decided by the target triple, not whether `--gcc-toolchain=` or `--gcc-install-dir=` is specified or not, or whether `crt0.o` (where does this file come from?) is present or absent.

I have a feeling that RISCVToolchain.cpp only works for certain vendors and other vendors cannot use it, and some builtin rules in RISCVToolchain.cpp are not clear about the choices.

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