powerpc: Make rwlocks endian safe

Our ppc64 spinlocks and rwlocks use a trick where a lock token and
the paca index are placed in the lock with a single store. Since we
are using two u16s they need adjusting for little endian.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>