x86/compat: provide optimized memcpy_x86

Implementation for PIE's builtin_memcpy located at
criu/arch/x86/include/asm/string.h

travis-ci: success for Compel/compat cleanups
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>