[lld-macho] Switch to xxh3_64bits

xxh3 is substantially faster than xxh64.
For lld/ELF, there is substantial speedup in `.debug_str` duplicate
elimination (D154813). Use xxh3 for lld-macho as well.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D155677