In debug mode iOS builds, we build a stub App.framework with no
functionality, since the engine itself loads the code from the included
bundle and subsequently via hot reload. This is simply done for
consistency with profile/release AOT app structure.

To satisfy the linker, ensure that we're building this code for all
build architectures, not just CURRENT_ARCH.

This is pre-work for supporting arbitrary iOS architectures (armv7,
arm64, universal) in Flutter.

This re-lands feb16d8d, which was reverted due to a flaky test.