iOS cross build on MacOS (#3699)
* Enable iOS cross build on MacOS (step#1)
* Changed parallel option
* fixed style issues
* Enable ios arm64 crossbuild on MacOS
* Enable ios arm64 crossbuild on MacOS
* Enable parallel build for xcode
* Fix arm64 function not 4-byte aligned warning
* Rename onnxruntime_ios.cmake to onnxruntime_ios.toolchain.cmake
* change build.py to use the new ios toolchain file name
