tools/nolibc: avoid -Wundef warning for __STDC_VERSION__

tools/nolibc: avoid -Wundef warning for __STDC_VERSION__

With -std=c89 the macro __STDC_VERSION__ is not defined.
While undefined identifiers in '#if' directives are assumed to be '0',
with -Wundef a warning is emitted.

Avoid the warning by explicitly falling back to '0' if __STDC_VERSION__
is not provided by the preprocessor.

Fixes: 37219aa5b123 ("tools/nolibc: add __nolibc_static_assert()")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260318-nolibc-wundef-v1-1-fcb7f9ac7298@weissschuh.net