tools/nolibc: process open() vararg as mode_t

openat() uses mode_t for this, so also update open() to be consistent.

Signed-off-by: Louis Taylor <louis@kragniz.eu>
Link: https://lore.kernel.org/r/20250306184147.208723-3-louis@kragniz.eu


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>