powerpc/32: Few optimisations in memcpy

This patch adds a few optimisations in memcpy functions by using
lbzu/stbu instead of lxb/stb and by re-ordering insn inside a loop
to reduce latency due to loading

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>