Use symbol file to export symbols in onnxruntime library on AIX platform as well

[ayappanec]

On AIX platform , the exporting of symbols in libonnxruntime library is handled through onnxruntime_c_api.cc and CMake itself. Because of which it exports lot of symbols but misses out OrtSessionOptionsAppendExecutionProvider_CPU.

This change is to make the symbol exports on AIX platform similar to how it is done for other platforms.

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