Apple's ld emit N_BNSYM and N_ENSYM to mark the start and end of
functions, while ld64.lld doesn't.
This resulted in MachOFile.load bailing out on unsupported STABS
layout when the linker used is ld64.lld.
This commit supports both layouts.
Like ELF, we now have `std.debug.MachOFile` for the host-independent
parts, and `std.debug.SelfInfo.MachO` for logic requiring the file to
correspond to the running program.