libc: Suppress <stdalign.h> content for C23 and later

C23 deprecates <stdalign.h> and specifies that the header shall
provide no content (§7.15.1).

Signed-off-by:	Faraz Vahedi <[email protected]>
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2223
MFC after:	1 month
Reviewed by:	imp, fuz

include/stdalign.h

@@ -26,6 +26,8 @@
  * SUCH DAMAGE.
  */
 
+#if __STDC_VERSION__ < 202311L
+
 #ifndef __alignas_is_defined
 #define	__alignas_is_defined	1
 
@@ -45,3 +47,5 @@
 #endif
 
 #endif /* !__alignof_is_defined */
+
+#endif /* __STDC_VERSION__ < 202311L */