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

[Peter Smith via cfe-commits]

smithp35 wrote:

> > 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.
> 
> Thanks for providing feedback @smithp35. I can add documentation related to the points you mentioned. Pls let me know if adding them in the BareMetal.cpp file would be ok or should I consider adding it in some other file.

I think developer documentation in BareMetal.cpp is fine. User documentation is probably best in the clang/docs, for example updating https://github.com/llvm/llvm-project/blob/main/clang/docs/Toolchain.rst 

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