syscalls: Introduce sys_kcmp syscall

Though we can use libc's syscall() wrapper
I would prefer to have own implementation
in case if we will need it in non-libc bindable
code.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>