[JITLink][MachO] Use correct symbol scope when N_PEXT is set and N_EXT unset.

MachOLinkGraphBuilder has been treating these as hidden, but they should be
treated as local.

Symbols with N_PEXT set and N_EXT unset are produced when hidden symbols are
run through 'ld -r' without passing -keep_private_externs. They will show up
under 'nm -m' as "was private extern", hence the name of the test cases.

Testcase commited as relocatable object to ensure that the test suite doesn't
depend on having 'ld -r' available.