[lld/mac] simplify export-options.s test a bit

- the macro seems needlessly clever -- shorter and imho clearer without it
- give all filenames an extension so they look like filenames
- rename .private_extern symbol from _private to _private_extern
  to prepare for follow-up that adds a truly private symbol

No behavior change.

Differential Revision: https://reviews.llvm.org/D101222