Xcode: Make sure QMAKE_PRE_LINK dependencies are complete for multi-arch builds

With multi-architecture builds and ONLY_ACTIVE_ARCH set to NO, Xcode will
build the final target for multiple architectures at the same time, but
CURRENT_ARCH will only match one of them, so we failed to set up the
right dependencies for our pre-link step, causing the step to happen
after linking in some cases.

We now build an exhaustive dependency list based on QMAKE_XCODE_ARCHS,
so that ONLY_ACTIVE_ARCH=NO can be used for release builds targeted at
the App Store.

Change-Id: I6702f020a6970807adc624779f6dde09be62beb9
Reviewed-by: Andy Shaw <andy.shaw@digia.com>