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

[Petr Hosek via cfe-commits]

petrhosek wrote:

> 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.

@nickdesaulniers is on paternity leave but he didn't raise any concerns, @PiJoules is familiar with the changes.

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