xtensa: fix memmove(), bcopy(), and memcpy().

- fix memmove to correctly handle overlapping src and dst;
- fix memcpy loop ending conditions from signed '<=' to '!=';
- modify bcopy to call memmove;

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>