[lld/mac] Parallelize code signature computation

According to ministat, this is a small but measurable speedup
(using the repro in PR56121):

    N           Min           Max        Median           Avg        Stddev
x  10     3.7439518     3.7783802     3.7730219     3.7655502   0.012375226
+  10     3.6149218      3.692198     3.6519327     3.6502951   0.025905601
Difference at 95.0% confidence
	-0.115255 +/- 0.0190746
	-3.06078% +/- 0.506554%
	(Student's t, pooled s = 0.0203008)

(Without 858e8b17, this change here to use parallelFor is an 18% speedup,
and doing 858e8b17 on top of this change is just a 2.55% +/- 0.58% win.
Doing both results in a total speedup of 20.85% +/- 0.44%.)

Differential Revision: https://reviews.llvm.org/D128298