builtin_{memcpy,memset}: make compatible with libc

1. memcpy() last argument is size_t
2. memset() returns void *

Make our builtin_*() functions compatible.

https://travis-ci.org/kolyshkin/criu/builds/174634847


Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>