Support Apple Silicon ARM simulators

[jmagman]

The simulator binary is missing the arm64 slice needed to run on Apple Silicon arm64 simulators.

ios-i386_x86_64-simulator/FirebaseFirestore.framework/FirebaseFirestore: Mach-O universal binary with 2 architectures: [i386:current ar archive] [x86_64]
ios-i386_x86_64-simulator/FirebaseFirestore.framework/FirebaseFirestore (for architecture i386):	current ar archive
ios-i386_x86_64-simulator/FirebaseFirestore.framework/FirebaseFirestore (for architecture x86_64):	current ar archive

I think it creates a ios-arm64_x86_64-simulator directory in the bundle though I'm not 100% sure on the name.

[jmagman]

Note CocoaPods currently has ffi issues CocoaPods/CocoaPods#9907. You can test by directly embedding the xcframework, targeting a simulator, and swap ONLY_ACTIVE_ARCH to NO.

[Salakar]

Tracking upstream; firebase/firebase-ios-sdk#6520

[mikehardy]

@jmagman I think this may work now, the upstream issue is closed. Can you confirm current status with current version (v7.2.0 should work but v7.3.0 just came out and is current now)