[lld-macho][objc] Don't crash when rodata isn't statically linked

We were previously assuming that to hold, but it appears that `swiftc`
may generate classes that link against `__objc_empty_cache` for their
rodata pointer.

This should allow us to re-land {D142916} (as a stack).

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D147229