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Having this as a freestanding argument meant that it was required if
you wanted to pass any arguments to lit, since parse_known_args meant
that the first unrecognized argument was always assumed to be a path
to the NDK.

Test: ./test_libcxx.py --param=build_only=True
Test: ./test_libcxx.py --ndk foo
Test: ./test_libcxx.py --ndk foo --param=build_only=True
Bug: None
Change-Id: If93e2fceefeb093ad60df50f7125701636652871