tools/nolibc: fix spelling of FD_SETBITMASK in FD_* macros

While nolibc-test does test syscalls, it doesn't test as much the rest
of the macros, and a wrong spelling of FD_SETBITMASK in commit
feaf7565 broke programs using either FD_SET() or FD_CLR() without
being noticed. Let's fix these macros.

Fixes: feaf7565 ("nolibc: fix fd_set type")
Cc: stable@vger.kernel.org # v6.2+
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>