meson: Let Panfrost's standalone compiler build on macOS
No, this isn't for a Pinebook Pro Hackintosh. (Although there are arm64 binaries of the whole system now...)
This is enough to exercise the compiler and its unit tests on macOS, which is good for when I get cranky waiting for my Chromebook to build and for mainline to support the M1.
There's nothing macOS specific here, this probably lets the Mali toolchain build under Windows, but I refuse to test that.