[clang] [Driver] Support using toolchain libc and libc++ for baremetal (PR #96736)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

The description can specify the target triple you are adding support for.
LGTM, but other folks more familiar with your planned changes need to stamp as well.

https://github.com/llvm/llvm-project/pull/96736