[lld/mac] Use normal Undefined machinery for dyld_stub_binder lookup

This is for aesthetic reasons, I'm not aware of anything that needs
this in practice. It does have a few effects:

- `-undefined dynamic_lookup` now has an effect for dyld_stub_binder.
  This matches ld64.

- `-U dyld_stub_binder` now works like you'd expect (it doesn't work in ld64).

- The error message for a missing dyld_stub_binder symbol now looks like
  other undefined reference symbols, it changes from

      symbol dyld_stub_binder not found (normally in libSystem.dylib). Needed to perform lazy binding.

  to

      error: undefined symbol: dyld_stub_binder
      >>> referenced by lazy binding (normally in libSystem.dylib)

Also add test coverage for that error message.

But in practice, this should have no interesting effects since everything links
in dyld_stub_binder via libSystem anyways.

Differential Revision: https://reviews.llvm.org/D105781