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

[Peter Smith via cfe-commits]

https://github.com/smithp35 commented:

I'm looking at this patch in isolation, so its possible that the comments here are addressed elasewhere.

It looks like useful functionality to add to the bare metal driver.

It also looks like there is some scope for some documentation, for users and for toolchain developers. Users will need to know about what options to set to point at their GCC installation (or place adjacent). Non-GNU toolchain developers will need to avoid `<triple>/lib/crt0.o`. If multilibs doesn't work for all toolchains yet, would be good to add that as a limitation.

That could be done with a follow up patch though.

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