[lld-macho] Parallelize UUID hash computation

This reuses the approach (and some code) from LLD-ELF.

It's a decent win when linking chromium_framework on a Mac Pro (3.2 GHz 16-Core Intel Xeon W):

      N           Min           Max        Median           Avg        Stddev
  x  20          4.58          4.83          4.66        4.6685   0.066591844
  +  20          4.42          4.61           4.5         4.505    0.04751731
  Difference at 95.0% confidence
          -0.1635 +/- 0.0370242
          -3.5022% +/- 0.793064%
          (Student's t, pooled s = 0.0578462)

The output binary is 381MB.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D99279