[clang] [CMake][Fuchsia] Build libc++ on top libc for baremetal (PR #99009)

[Paul Kirth via cfe-commits]

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

Not a fan of this approach, but, seeing as it’s temporary and only affects our builds, I suppose it’s fine for now. 

https://github.com/llvm/llvm-project/pull/99009