stddef.h: don't include sys/cdefs.h

Switch to sys/_visible.h for visibility macros.

Prefer __builtin_offsetof over __offset.  sys/cdefs.h always defines
__offsetof to __builtin_offsetof so just use the latter to remove a
dependency on sys/cdefs.h.  Realistically, we're never going to care
about a compiler that doesn't supply this builtin.

Add a somewhat questionable guard around the offsetof() definition
because the compiler no longer thinks it the same as a number of other
redundent definitions scattered around (e.g., in the openzfs codebase).
It is infact the same and those defintions likely shouldn't exist at
all.

include/stddef.h

@@ -32,9 +32,9 @@
 #ifndef _STDDEF_H_
 #define _STDDEF_H_
 
-#include <sys/cdefs.h>
 #include <sys/_null.h>
 #include <sys/_types.h>
+#include <sys/_visible.h>
 
 #ifndef _PTRDIFF_T_DECLARED
 typedef	__ptrdiff_t	ptrdiff_t;
@@ -61,7 +61,9 @@ typedef	__max_align_t	max_align_t;
 #endif
 #endif
 
-#define	offsetof(type, field)	__offsetof(type, field)
+#ifndef offsetof
+#define	offsetof(type, field)	__builtin_offsetof(type, field)
+#endif
 
 #if __EXT1_VISIBLE
 /* ISO/IEC 9899:2011 K.3.3.2 */