ios: override spec for ios

On ios qmake is called recursively and the second call with a different
spec (macx-xcode instead of macx-ios-clang) which contains the
correct build settings (includes, compiler flags) as ios builds with
xcode.
macx-ios-clang just creates supporting makefiles, and to avoid being
slow does not evaluate everything, and contains misleading information
(that is never used), whereas macx-xcode correctly evaluates the
the variables and generates the xcodeproject that is actually used to
build the application.

It is important to override only for the creator evaluator, and not
the qmake buildstep used to build the app (as we use the makefiles).

Task-number: QTCREATORBUG-11908
Change-Id: I06d569de16f934fca5e104a8da727a3557a4c2e3
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>