We want to compress DWARF even on macOS, but the native toolchain isn't
going to understand it. Add a flag that can be used to disable
compression, then add Darwin to the whitelist used during internal
linking.
Unlike GNU ld, the Darwin linker doesn't have a handy linker flag to do
compression. But since we're already doing surgery to put the DWARF in
the output executable in the first place, compressing it at the same
time isn't unduly difficult. This does have the slightly odd effect of
compressing some Apple proprietary debug sections, which absolutely
nothing will understand. Leaving them uncompressed didn't make much
sense, though, since I doubt they're useful without (say) __debug_info.