The same constants and some more have been exposed to userspace via
linux/reboot.h for a long time.

To avoid conflicts and trim down nolibc a bit drop the custom
definitions.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>