Fallback implementation of Q_ALIGNOF

Fallback implementation of Q_ALIGNOF

For all practical purposes, the fallback introduced here returns the
desired value. Having a fallback enables unconditional use of Q_ALIGNOF.

For compilers that provide native support for it, Q_ALIGNOF is otherwise
 #defined in qcompilerdetection.h.

Change-Id: Ie148ca8936cbbf8b80fe87771a14797c39a9d30c
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>