cdefs.h: Introduce __nonstring attribute

This attribute can be used to annotate char arrays that are not supposed to
be terminated with a NUL char and is needed to silence clang's new
-Wunterminated-string-initialization warning. The name matches linuxkpi.

Reviewed by:	emaste, jhb
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D52565

(cherry picked from commit 802c6d5d61d15494a54a386dc2ffbcfefc68c8ab)

sys/sys/cdefs.h

@@ -521,6 +521,13 @@
 #define	__printf0like(fmtarg, firstvararg)
 #endif
 
+/* To silence warnings about null terminator not fitting into an array. */
+#if __has_attribute(__nonstring__)
+#define	__nonstring	__attribute__((__nonstring__))
+#else
+#define	__nonstring
+#endif
+
 #if defined(__GNUC__)
 #define	__strong_reference(sym,aliassym)	\
 	extern __typeof (sym) aliassym __attribute__ ((__alias__ (#sym)))

tools/build/cross-build/include/common/sys/cdefs.h

@@ -100,6 +100,14 @@
 	__attribute__((__format__(__printf0__, fmtarg, firstvararg)))
 #endif
 
+#ifndef __nonstring
+#if __has_attribute(__nonstring__)
+#define	__nonstring	__attribute__((__nonstring__))
+#else
+#define	__nonstring
+#endif
+#endif
+
 #ifndef __predict_true
 #define __predict_true(exp) __builtin_expect((exp), 1)
 #endif