[test] llvm-ar: Adjust 2 tests to pass again after inadvertent unXFAIL for some target triples (#159796)
The problem with the current `target={{.*}}-darwin{{.*}}` XFAIL
condition (changed in https://github.com/llvm/llvm-project/pull/130144)
is that the Swift build script uses Apple platform names like 'macosx',
not 'darwin', for the LLVM target triple, e.g. `arm64-apple-macosx13.0`.
These tests appear to have been originally XFAILed because the default
format on macOS (darwin) adds newlines as padding to align members. See:
https://github.com/llvm/llvm-project/blob/ee8394d9469a2946ffe2e7d192c593ecf3f93098/llvm/lib/Object/ArchiveWriter.cpp#L897-L904
Use `--format=gnu` to cope with the differences in the output and
reenable the tests.
rdar://157213658