tools/nolibc: add _exit()

_exit() is the faster variant of exit(), skipping all cleanup actions.
As nolibc does not perform any cleanup anyways, the implementation is
trivial.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>