Mach-O: fill up `LC_UUID` command

[marxin]

Similarly to ELF's .build-id, we should fill up hash based on the content of the emitted file.

[joanna-jo]

Hello! I had a lot of fun working on the last issue and wanted to ask if this would be a good next issue to try for the Mach-O port? I’m not too familiar with hashing algorithms, but I noticed wild currently uses BLAKE3 for ELF's .build-id and was wondering whether it sounded reasonable to implement LC_UUID with a truncated BLAKE3 hash as well.

[davidlattimore]

Generally for hashing, I'd say whatever is fastest. I don't recall how we ended up with BLAKE3, but using it as a starting point for LC_UUID sounds good to me. If we discover something else is faster, we can look to switch both ELF and MachO.