[lld-macho] Parallelize __LINKEDIT generation

Benchmarking chromium_framework on a 3.2 GHz 16-Core Intel Xeon W Mac Pro:

      N           Min           Max        Median           Avg        Stddev
  x  20          4.33          4.42          4.37          4.37   0.021026299
  +  20          4.12          4.23          4.18         4.175   0.035318103
  Difference at 95.0% confidence
    -0.195 +/- 0.0186025
    -4.46224% +/- 0.425686%
    (Student's t, pooled s = 0.0290644)

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D99998