proposal: x/mobile: add a way to specify IPHONEOS_DEPLOYMENT_TARGET for maccatalyst

[fortuna]

Currently if you specify the flag -iosversion if affects both the ios and maccatalyst frameworks. However, I need target 11.0 for ios and 13.1 for maccatalyst, because maccatalyst requires 13.1, but I don't want to raise the version for ios and lose users.

I need a way to specify different versions for the different platforms.

It may be worth it to provide a way to specify our own build config files. I believe I would be able to set IPHONEOS_DEPLOYMENT_TARGET[sdk=macosx*]=13.1 on a build config, though that doesn't work as an env variable.

[bcmills]

(CC @golang/ios)