[lld-macho] Treat undefined symbols uniformly

In particular, we should apply the `-undefined` behavior to all
such symbols, include those that are specified via the command line
(i.e.  `-e`, `-u`, and `-exported_symbol`). ld64 supports this too.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D102143