Support macOS with Apple Silicon

[Shakarang]

Apple will release a new type of Mac having a different architecture than x86_64. This architecture is called Apple Silicon (arm64) and requires projects with custom VALID_ARCHS to add it in their VALID_ARCHS.

Checklist

• This is not a Apple's bug.
• Reviewed the README and documents.
• Searched existing issues for ensure not duplicated.

Expected Behavior

DifferenceKit can be used on macOS having the Apple Silicon architecture.

Current Behavior

The Apple Silicon architecture (arm64) is not in the valid architectures of the project.

Steps to Reproduce

Build the framework and use the lipo -info command to see that the architecture is missing.

[taoshotaro]

Since VALID_ARCHS is not used by default in Xcode 12, isn't it better to remove VALID_ARCHS from the xcconfig?

[ra1028]

Fixed #124