Add initial support for Apple Silicon (#27264)

* Add initial support for Apple Silicon

- Add osx-arm64 runtime dependency
- Add osx-arm64 to supported rid identifiers
- Update vmImage to macOS-10.15
- Use XCode 12.2 for all macOS builds
- Add build and sign macos arm64 to CI scripts

This should build aspnetcore for osx-arm64, however
the native dependencies libuv and libe_sqlite3 will
need to be updated to have full functionality.  These
are provided by separate repos.

Enabling at this point unblocks effort to create the initial
osx-arm64 SDK.

* PR Feedback