Support arm64 target for builds for iOS Simulator

Qt builds for iOS currently generate binaries for the x86_64 target for the iOS Simulator. At the moment it does not cause problems in app development in testing in the iOS Simulator even on an Apple Silicon based machine. MacOS starts the iOS Simulator as expected and executes it that the app runs in it as expected.

If and when in some future versions of MacOS the platform would expect native (arm64) binaries for the iOS Simulator running on Apple Silicon, apps built for x86_64 iOS Simulator target will fail to run. Apple's plans on this are not known at the time of writing, but it is possible that this might happen even earlier than Rosetta will be discontinued since for most of the developers it does not matter which CPU architecture the OS Simulator is simulating as long as it behaves as expected in a simulator. 

For me, it sounds like one of those problems which we should take care of before it actually occurs