PPC: E500: Inject SPE exception on invalid SPE access

When accessing an SPE instruction despite it being not available,
throw an SPE exception instead of an APU exception. That way the
guest knows what's going on and actually uses SPE.

Reported-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
