Use Xcode with support for universal binaries

[MaxDesiatov]

After swiftlang#32706 has been merged upstream (into both release/5.3 and master), the host SDK can no longer be compiled with GitHub Actions, since Xcode 12 beta installed on those action runners doesn't support building for arm64.

This is currently blocked by actions/runner-images#1166.

[MaxDesiatov]

Per a discussion in Discord, we'll probably start with separate distribution packages for each platform. This is easier to set up, and carton will download the appropriate package automatically. This can be reopened if a good use case for universal binaries is found that outweighs the maintenance overhead.