ppc64: Use optimized memcpy

Instead of belonging to the common C memcpy function, belong on the
optimized one stolen from the kernel.

Cc: Anton Blanchard <anton@au.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>