Investigate building osx-arm64 on osx-x64

[sdmaclea]

It seems like it is feasible to build osx-arm64 on a osx-x64 build platform.

Investigate and see what needs to be done.

This will prepare us for #41131

[sdmaclea]

I think this should be close to working in #40435

Per XCode release notes, we seem to need at least XCode 12 Beta *. Which requires

Xcode 12 beta 6 requires 
Apple silicon running macOS Big Sur 11 beta or later, 
or an Intel-based Mac running macOS Catalina 10.15.4 or later

I'll try this upgrade week when I have stable WiFi....

[sdmaclea]

@dotnet/osx-arm64-core This is the github alias I created.

[janvorli]

This was already done in #43187