Implement macOS dispatch semaphores

[RalfJung]

rust-lang/rust#102773 has started using macOS dispatch semaphores for thread parking on macOS. We should implement those APIs in Miri. (Right now a cfg(miri) makes miri use a pthread-based fallback implementation.)

These seem to be the API docs.

[thomcc]

Yes, those are the docs. Feel free to ping me if you have questions.

Note that its lifetime is managed by dispatch_retain/dispatch_release calls, which manage an atomic reference count. We only use dispatch_release in std patch (since the object returned by dispatch_create has a retain count of 1 initially), and I don't see a reason in the near term we'd use dispatch_retain for anything, but I'd advise against implementing this in a way that adding the refcount later would require major hacks or something, not that that seems likely.

[RalfJung]

rust-lang/rust#122408 may switch the macOS primitives to yet another OS API, so I am not sure if it would currently be worth it to implement dispatch semaphores.