ffmpeg: macOS uses "_" symbol prefix (Mach-O), has no <malloc.h>,
and doesn't support .data.rel.ro sections. Add __APPLE__ guards
in ffmpeg-xenia/config.h.
sha256: macOS provides endian macros in <machine/endian.h> instead
of <endian.h>.
Based entirely off existing xbyak x86 implementation and available
tests. Still needs a lot of optimization and testing on non-Windows
platforms.
So far passes all tests and boots at least some games on Windows.
