PPC: Add STLU

For assembly code there are several "long" load and store defines already.
The one that's missing is the typical stack store, stdu/stwu.

So let's add that define as well, making my KVM code happy.

CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Avi Kivity <avi@redhat.com>