Skip buffer contents comparison in bitcast_op_test_cpu for s390x
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After PR https://github.com/tensorflow/tensorflow/pull/57069, **bitcast_op_test_cpu** test case started failing for BE machines due to difference in buffer contents.

As per TensorFlow [bitcast](https://www.tensorflow.org/api_docs/python/tf/bitcast#:~:text=Bitcast%20is%20implemented%20as%20a%20low%2Dlevel%20cast%2C%20so%20machines%20with%20different%20endian%20orderings%20will%20give%20different%20results.%20A%20copy%20from%20input%20buffer%20to%20output%20buffer%20is%20made%20on%20BE%20machines%20when%20types%20are%20of%20different%20sizes%20in%20order%20to%20get%20the%20same%20casting%20results%20as%20on%20LE%20machines.) documentation, buffer contents after bitcast operation may differ for BE machines, causing **bitcast_op_test_cpu** to fail.

Hence, skipping buffer content comparison/assert done in bitcast_op_test_cpu for s390x(big-endian). 

This code change would not cause any regressions on existing test cases and it won't affect LE machines.

Will rectify below test case for s390x:
`//tensorflow/python/kernel_tests/array_ops:bitcast_op_test_cpu`

Signed-off-by: Yasir Ashfaq <Yasir.Ashfaq1@ibm.com>