It seems like we are overly asserting when running `-dead_strip` on on
exported symbols. ld64 links successfully with `-dead_strip` even when
ld64 links successfully with `-dead_strip` even when a symbol is cannot be exported (i.e. `weak_def_can_be_hidden` and
`weak_def_can_be_hidden` and ``private_extern`). Loosen the assert to match ld64's
behavior .