[lld-macho] Avoid explicit -arch in tests by defaulting to x86-64

Summary:
As mentioned in https://reviews.llvm.org/D81326#2093931, I'm not sure it
makes sense to use the default target triple to determine -arch.
Long-term we should probably detect it from the input object files, but
in the meantime it would be nice not to have to add it to all our tests
by using a convenient default.

Reviewers: #lld-macho

Subscribers: arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81983