tools/nolibc: fold llseek fallback into lseek()

Align the implementation of the fallback handling inside sys_lseek()
with the rest of nolibc.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250821-nolibc-enosys-v1-5-4b63f2caaa89@weissschuh.net