Fix automatic mkspec setting for iOS simulator builds

Adds macx-ios-clang to the list of "suggested" mkspecs for Clang/Mac.

Change-Id: I3c3f803d651c42e1c2ed7a0f77047a531acf6a0c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>