tools/nolibc: automatically detect necessity to use pselect6

We can automatically detect if pselect6 is needed or not from the kernel
headers. This removes the need to manually specify it.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20230917-nolibc-syscall-nr-v2-4-03863d509b9a@weissschuh.net