[lld-macho] No need to explicitly specify -arch in tests

Summary: After {D81326} landed, some tests started failing if they did
not have `-arch` specified. I think one of the reasons happened was due
to the fact that we were taking a reference to a temporary value that
was freed too early. Fixing that got the error to go away on my local
Linux machine.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D81802