headers: Move prctl constants to prctl.h

Move prctl constants into own header. They are
arch independent.

The x86 specific ARCH_SET_GS should be fetched from <asm/prctl.h>
(instead of defining own ones).

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