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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

If everything is already running on machines owned by the foundation, it should just be a matter of setting up the Github actions agent on them to get more runner availability? Like @tstellar mentioned, that's not what this PR does currently though.

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