[clang] [RISCV] Merging RISCVToolChain and BareMetal toolchains (PR #118809)

[Petr Hosek via cfe-commits]

petrhosek wrote:

The `BareMetal` driver currently isn't very idiomatic (compared to other drivers like `Generic_GCC`) and duplicates a lot of the logic that has been already factored out in other drivers. I think this change could be made a lot smaller by doing a clean up first, bringing the `BareMetal` driver closer to other drivers, before attempting to merge the `RISCVToolChain` driver into it. I already landed https://github.com/llvm/llvm-project/pull/101259 which is a first step in that direction and I plan to make a few more follow up cleanup changes.

https://github.com/llvm/llvm-project/pull/118809