[POWERPC] Fix return value from memcpy

As pointed out by Herbert Xu <herbert@gondor.apana.org.au>, our
memcpy implementation didn't return the destination pointer as its
return value, and there is code in the kernel that expects that.
This fixes it.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>