iOS: Tell Xcode to not build universal app binaries for debug builds

You're likely to only target/develop on one device at a time, so
we only need to build for one architecture at a time. Switching
device in Xcode will switch the active architecture as well, so
the only case where you'll need a universal debug build is if
you are creating a debug package for testers.

Change-Id: I4f37f5c982082c42836749d1e9fbe5ef91138912
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>