Update build.md for building using Xcode 12 on Mac (#5256)
Update build.md for building using Xcode 12 on Mac (#5256)

* update build.md

* update build.md

* Address pr comments
microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime