For embedded we do not want to override the mkspec as used in the Qt version
since that was most likely tweaked to work with the tool chain selected.

In a generic desktop target we want to be able to switch between different
compilers of the same ABI.

Change-Id: Ib49e54dd15e78c2459b17c552a3c31cb3dce63eb
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>