[lld-macho][nfc] Fix up missing FileCheck on objc.s

I'll be making changes to the ObjC implementation (bug fix) in a later
change, and I noticed that this test isn't being tested at all. Fix that up
here separately so further changes are easier to differentiate.

Reviewed By: #lld-macho, thakis, int3

Differential Revision: https://reviews.llvm.org/D139396