lldb: Run TestCrossDSOTailCalls.py and TestCrossObjectTailCalls.py on Darwin only

See https://bugs.llvm.org/show_bug.cgi?id=44561, these tests are failing
on an aarch64/Linux bot:

http://lab.llvm.org:8011/builders/lldb-aarch64-ubuntu/builds/655

For some reason the backtrace the tests are expecting to find is
incomplete.