[lld/mac] Make X86_64::getImplicitAddend not do heap allocations

Speeds up linking Chromium's base_unittests almost 10%. According to ministat:

    N           Min           Max        Median           Avg        Stddev
x   5    0.72193289    0.73073196    0.72560811    0.72565799  0.0032265649
+   5    0.64069581    0.67173195    0.65876389    0.65796089   0.011349451
Difference at 95.0% confidence
	-0.0676971 +/- 0.0121682
	-9.32906% +/- 1.67685%
	(Student's t, pooled s = 0.00834328)

Differential Revision: https://reviews.llvm.org/D92734