MLAS: Add MlasComputeSoftmax/MlasComputeExp (#3846)
* add MlasComputeSoftmax
* fix onnxruntime_mlas_test DLLs
* remove unneeded header
* remove unneeded header
* call MlasComputeExp
* call MlasComputeSoftmax
* call MlasComputeSoftmax
* finish off
* fix static analysis warning
