Use alias function attribute to provide memcpy(), memcmp()
and memset() in case we're not linking with libc. This is
needed as compiler can generate calls to those functions
(clang actually does that).

[v2: add comment, move aliases to under HAS_BUILTIN_*]

https://travis-ci.org/kolyshkin/criu/builds/174634847Idea-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>