[lld/mac] Set ordinal on dynamic undefined symbols in symbol table

This lets `nm -m` print "(from libfoo)" in its output, which is more
accessible than dumping the bind table.

See https://reviews.llvm.org/D57190#2455761 for the somewhat
surprising `AltEntry` that appears in symtab.s.

Differential Revision: https://reviews.llvm.org/D93318