[LLD][ELF][ARM] Don't apply --fix-cortex-a8 to relocatable links.

The --fix-cortex-a8 is sensitive to alignment and the precise destination
of branch instructions. These are not knowable at relocatable link time. We
follow GNU ld and the --fix-cortex-a53-843419 (D72968) by not patching the
code when there is a relocatable link.

Differential Revision: https://reviews.llvm.org/D73100