[lldb] Skip TestCortexMExceptionUnwind except Darwin
I'm getting a failure on one linux CI, lldb-remote-linux-ubuntu,
where the test of assertEqual(thread.GetNumFrames(), 6) fails
because the unwinder only has one frame, most likely the
target triple is not set to armv7m-apple-* so ABISysV_arm is
being used instead of ABIMacOSX_arm and the architecture default
unwind plans behave differently. Possibly the frame pointer
register, or possibly the way the arch default unwind plans are
structured.
Skipping on linux for now until I can debug further. Can repo on
Darwin by changing the binary.json `"triple": "armv7m-apple"` to
armv7m-linux.
