tools/nolibc: add support for openat(2)

openat is useful to avoid needing to construct relative paths, so expose
a wrapper for using it directly.

Signed-off-by: Louis Taylor <louis@kragniz.eu>
Link: https://lore.kernel.org/r/20250306184147.208723-1-louis@kragniz.eu


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