stddef.h: centralize definition of offsetof()

Move to sys/_offsetof.h and use __builtin_offsetof() instead of
__offsetof to avoid reintroducing sys/cdefs.h polution in stddef.h.

This has the side effect of allowing sys/stddef.h to be included after
stddef.h which can happen in compatability headers.

Effort:		CHERI upstreaming
Sponsored by:	DARPA, AFRL
Reviewed by:	imp, kib
Differential Revision:	https://reviews.freebsd.org/D55307

include/stddef.h

@@ -33,6 +33,7 @@
 #define _STDDEF_H_
 
 #include <sys/_null.h>
+#include <sys/_offsetof.h>
 #include <sys/_types.h>
 #include <sys/_visible.h>
 
@@ -68,10 +69,6 @@ typedef	__max_align_t	max_align_t;
 #endif
 #endif
 
-#ifndef offsetof
-#define	offsetof(type, field)	__builtin_offsetof(type, field)
-#endif
-
 #if __EXT1_VISIBLE
 /* ISO/IEC 9899:2011 K.3.3.2 */
 #ifndef _RSIZE_T_DEFINED

sys/sys/_offsetof.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2026 Capabilities Limited
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * This software was developed by SRI International, the University of
+ * Cambridge Computer Laboratory (Department of Computer Science and
+ * Technology), and Capabilities Limited under Defense Advanced Research
+ * Projects Agency / Air Force Research Laboratory (DARPA/AFRL) Contract
+ * No. FA8750-24-C-B047 ("DEC").
+ */
+#ifndef _SYS__OFFSETOF_H_
+#define	_SYS__OFFSETOF_H_
+
+#ifndef offsetof
+#define offsetof(type, field)   __builtin_offsetof(type, field)
+#endif
+
+#endif /* _SYS__OFFSETOF_H_ */

sys/sys/stddef.h

@@ -31,6 +31,7 @@
 
 #include <sys/cdefs.h>
 #include <sys/_null.h>
+#include <sys/_offsetof.h>
 #include <sys/_types.h>
 #include <sys/_visible.h>
 
@@ -46,6 +47,4 @@ typedef	__ptrdiff_t	ptrdiff_t;
 #define	_PTRDIFF_T_DECLARED
 #endif
 
-#define	offsetof(type, field)	__offsetof(type, field)
-
 #endif /* !_SYS_STDDEF_H_ */