[lld-macho][nfc] Centralize usages of ld64.lld in tests

We have a mix of substituted lld (`%lld`) and hard-coded lld (`ld64.lld`) commands.
When testing with different versions of LLD, this would require going into every place
where lld is hard-coded and changing that. If we centralize it, this'll only  require us
to modify it in only one place and will make it easy to run the same test suite. Plus,
this will make it be consistent with how we write other tests.

Reviewed By: #lld-macho, int3, oontvoo

Differential Revision: https://reviews.llvm.org/D119394