[libcxx] [llvm] [libc++] Move some macOS CI jobs to Github actions (PR #89083)

[via llvm-commits]

EricWF wrote:

Let's wait for the results to come back, but this LGTM.
Are we using the Github hosted machines for these builds now? If so, we probably need permission from the LLVM org before we start eating minutes.

https://github.com/llvm/llvm-project/pull/89083