[AIX] Disable clang python binding tests

Summary:
The Python ctypes FFI interface is broken on AIX, it cannot properly pass
structures containing  arrays ( https://bugs.python.org/issue38628). So
disable the clang python binding tests on AIX till this is resolved.

Reviewers: stevewan, jasonliu, hubert.reinterpretcast, mgorny

Reviewed By: jasonliu, hubert.reinterpretcast

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70675