Windows Phone and WinRT builds only support the x86_arm cross compiler.
With this change this behavior is auto detected by Qt Creator for these
builds (due to being higher prioritized). As soon as Windows Phone/WinRT
builds support cross compilation from x64, this change can be reverted.

Task-number: QTBUG-44530
Change-Id: I16ffe47c4582f51abdb6ef57b5922eb13863fd4e
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>