[clang] [Driver] Normalize the baremetal handling of libc++ and runtimes (PR #101259)

[Louis Dionne via cfe-commits]

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

LGTM. I don't own the driver, but this is how we want libc++ to be used (as generically as possible).

https://github.com/llvm/llvm-project/pull/101259