Add s390x support in ParseTensorOp and SerializeTensorOp
Hi @cantonios ,

I added a space before `"//tensorflow/core/util/tensor_bundle:byteswaptensor"` in `tensorflow/core/kernels/BUILD` as per the format checking result from the latest `Code Check - Changed Files`. Please take a look when you have some time.

Thank you very much! Hi @cantonios ,

Could you please take a look at the PR update when you have some time?

Thank you very much! Thanks @cantonios !
Sub-test `tensorflow.python.ops.io_ops.serialize_tensor` in test case `//tensorflow/tools/docs:tf_doctest` failed on s390x (big-endian arch), because the serialized binary strings which represent the tensor data are different between little-endian and big-endian platforms.
 
This PR adds the endianness conversion procedure for big-endian systems in the implementation of ParseTensorOp and SerializeTensorOp (in `tensorflow/core/kernels/parse_tensor_op.cc`), so that the serialized binary strings generated on big-endian systems will be in little-endian format as well and identical to the ones generated on little-endian systems.
 
The code change in `tensorflow/python/kernel_tests/io_ops/parsing_ops_test.py` ensures the serialized proto string in `ParseTensorOpTest.testToFloat32()` would be in little-endian format on big-endian platforms, which is consistent with the updated code flow in `SerializeTensorOp`.
 
`//tensorflow/tools/docs:tf_doctest` will pass on big-endian systems after applying the code change. It won't affect the functionality and performance on little-endian systems. 

This PR also updated the api documentation for `tf.io.serialize_tensor` and `tf.io.parse_tensor` to indicate the serialized binary strings are in little-endian format.

Signed-off-by: Kun-Lu <kun.lu@ibm.com>