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>

mkspecs/macx-ios-clang/features/default_post.prf

@@ -202,6 +202,11 @@ macx-xcode {
 
     QMAKE_MAC_XCODE_SETTINGS += arch_iphoneos arch_iphonesimulator
     QMAKE_XCODE_ARCHS = $$QMAKE_IOS_DEVICE_ARCHS $$QMAKE_IOS_SIMULATOR_ARCHS
+
+    only_active_arch.name = ONLY_ACTIVE_ARCH
+    only_active_arch.value = YES
+    only_active_arch.build = debug
+    QMAKE_MAC_XCODE_SETTINGS += only_active_arch
 } else {
     # Be more specific about which architecture we're targeting
     contains(QT_ARCH, arm.*): \