Add desktop PowerPC specific emulation

Little opcodes behave differently on desktop and embedded PowerPC cores.
In order to reflect those differences, let's add some #ifdef code to emulate.c.

We could probably also handle them in the core specific emulation files, but I
would prefer to reuse as much code as possible.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>