This change adds support for armv7, arm64, and universal iOS apps.

This change eliminates iOS target architecture hardcoding (previously
arm64 only) and uses the target architecture(s) specified in Xcode's
ARCHS setting ('Architectures' in Xcode Build Settings).

For universal binaries, set ARCHS to its default value, $(ARCHS_STANDARD).

Note that after changing the architecture in Xcode, developers should
run 'pod install' from the ios subdirectory of their project. A separate
change (that will land before this one) will add support for
automatically detecting project file and Podfile changes and re-running
pod install if necessary.

This change also adds an --ios-arch option to flutter build aot. In iOS
AOT builds (in profile and release mode), this dictates which
architectures are built into App.framework. This flag should generally
be unnecessary to set manually since flutter build aot is typically only
invoked internally by flutter itself.