asahi: Split off most of the macOS support into its own files

The macOS build is for testing purposes and it's likely that we will get rid of it in the future. However, if nothing else, it's useful to keep around as an option to merge in for future kernel interface experiments. Plus we do want to keep at least the libwrap stuff, since that is critical to continued reverse engineering of the shader/userspace portions.

As a first step, let's decouple most macOS support from the innards of the driver. This should make it easier to keep around or throw away in the future. After this MR, most macOS-only bits are no longer built on Linux and vice versa.