Add x86_64 for Intel-Mac support: macOS and iOS-simulator slices are now arm64_x86_64 fat. Devices stay arm64. tvOS simulator stays arm64-only (x86_64-apple-tvos needs nightly + build-std; Intel-Mac tvOS-simulator use not worth it). Five slices: macos-arm64_x86_64, ios-arm64, ios-arm64_x86_64-simulator, tvos-arm64, tvos-arm64-simulator.