tools/nolibc/types: make FD_SETSIZE configurable

The macro was hard-coded to 256 but it's common to see it redefined.
Let's support this and make sure we always allocate enough entries for
the cases where it wouldn't be multiple of 32.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>