[lld-macho] Devirtualize TargetInfo::getRelocAttrs

This method is called on each relocation when parsing input files, so
the overhead of using virtual functions ends up being quite large.  We
now have a single non-virtual method, which reads from the appropriate
array of relocation attributes set in the TargetInfo constructor.

This change results in a modest 2.3% reduction in link time for
chromium_framework measured on an x86-64 VPS, and 0.7% on an arm64 Mac.

    N           Min           Max        Median           Avg        Stddev
x  10     11.869417     12.032609     11.935041     11.938268   0.045802324
+  10     11.581526     11.785265     11.649885     11.659507   0.054634834
Difference at 95.0% confidence
	-0.278761 +/- 0.0473673
	-2.33502% +/- 0.396768%
	(Student's t, pooled s = 0.0504124)

Differential Revision: https://reviews.llvm.org/D130000