[libc++] Add new queues for specific macOS system versions

This will allow running back-deployment testing on macOS only on systems
running the right version of macOS. For the time being, we're cheating
because we don't have actual machines running older than 10.15.

GitOrigin-RevId: 3fb83853791cabd08fbc6cd35ed2b82c0ffcc2d6
2 files changed