[lld/mac] Implement -final_output

This is one of two flags clang passes to the linker when giving calling
clang with multiple -arch flags.

I think it'd make sense to also use finalOutput instead of outputFile
in CodeSignatureSection() and when replacing @executable_path, but
ld64 doesn't do that, so I'll at least put those in separate commits.

Differential Revision: https://reviews.llvm.org/D105449