`-mcpu` and `-code-model` tests were copied from similar ones in
LLD-ELF.
There doesn't seem to be an equivalent test for `-mattr` in LLD-ELF, so
I've verified our behavior by cribbing a test from
CodeGen/X86/recip-fastmath.ll.
Reviewed By: #lld-macho, compnerd, MaskRay
Differential Revision: https://reviews.llvm.org/D92912
Jez Ng