This is relevant to PIEs, which are notably enabled by default on macOS.
The build system needs to only see virtual addresses, that is, those
which do not have the slide applied; but the fuzzer itself naturally
sees relocated addresses (i.e. with the slide applied). We just need to
subtract the slide when we communicate addresses to the build system.
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.
