[AIX] test-suites failure fixes (#25791)
### Description
This PR is to fix some of the test case failures mentioned in
https://github.com/microsoft/onnxruntime/issues/25790
1. cmake/onnxruntime_unittests.cmake
To fix the onnxruntime_shared_lib_test crash
2. include/onnxruntime/core/providers/utils/ort_graph_to_proto.h
Added method for handling external data.
For fixing , EpGraphTest.SerializeToProto_ConstantOfShape ,
EpGraphTest.SerializeToProto_Mnist,
EpGraphTest.SerializeToProto_InputModelHasExternalIni
3. onnxruntime/core/framework/tensorprotoutils.cc
For fixing many test failures , related to float16 or Int16 type.
4. onnxruntime/test/framework/endian_test.cc
For fixing, ConvertRawDataInTensorProtoTest.FloatData and
ConvertRawDataInTensorProtoTest.Int32Data
### Motivation and Context
To fix the AIX (Big endian) related test failures.
