Add support for running on big-endian systems

Now qtmultimedia test suite passes on powerpc.

Change-Id: I540dff93195115ad1dc5725af7293e3b8540403f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>