[lld/mac] slightly improve weak-private-extern.s test

- __got is in --bind output, so print that too (makes the test
  a bit stronger)
- WEAK_DEFINES, BINDS_TO_WEAK are in the mach-o header, so
  --private-header is enough, no need for --all-headers
  (makes the test a bit easier to work with when it fails)

Differential Revision: https://reviews.llvm.org/D101065