[DWARFLinkerParallel] Switch to xxh3_64bits

xxh3 is substantially faster than xxh64.
For lld `.debug_str` there is substantial speedup (D154813).

@avl reports that it is around 1-2% improvement for different number of input sets and for different threads configuration.

Reviewed By: avl

Differential Revision: https://reviews.llvm.org/D155675