[lld-macho] Use fixed chunk size for UUID

Chunk size decided by the thread count makes the UUID less deterministic
(e.g. across machines with different core counts.)
Follow ELF and just use a fixed chunksize.

Fixes: https://github.com/llvm/llvm-project/issues/63961

Reviewed By: #lld-macho, keith

Differential Revision: https://reviews.llvm.org/D155761