tools/nolibc: validate C89 compatibility

To make sure no non-compatible changes are introduced accidentally
validate the language standard when building the tests.

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