PowerPC: memmove default implementation cleanup

This patch removes the powerpc specific logic in memmove and instead
include default implementation with MEMCPY_OK_FOR_FWD_MEMMOVE defined.
This lead in a increase performance, since the constraints to use
memcpy in powerpc code are too restrictive and memcpy can be used for
any forward memmove.

This is a backport of d6f68bbe.