[mach-o] Fix two-level namespace ordinals

On darwin, the linker tools records which dylib (DSO) each undefined was found
in, and then at runtime, the loader (dyld) only looks in that one specific
dylib for each undefined symbol.  Now that llvm-objdump can display that info
I can write test cases.
Add back a fallback case for targets that do not or cannot implement getNoopForMachoTarget().