[clang] [Driver] Teach Barmetal toolchain about GCC installation (PR #121829)

[Peter Smith via cfe-commits]

smithp35 wrote:

> Hi @smithp35, can you pls review the latest changes and approve this PR if everything looks fine?

I'll take a look this week. You'll probably need an approval from PetrHosek too. I'm mostly coming at this from an angle of will it break anything in the existing bare-metal driver. Probably needs some input from the RISCV side.

https://github.com/llvm/llvm-project/pull/121829