Move _Atomic() into <sys/cdefs.h>.

That way _Atomic() is defined next to all the other C11 keywords for
which we provide compatibility for pre-C11 compilers. While there, fix
the definition to place "volatile" at the end. Otherwise pointer types
will become "volatile T *" instead of "T * volatile".

sys/sys/cdefs.h

@@ -268,6 +268,14 @@
 #define	_Alignof(x)		__alignof(x)
 #endif
 
+#if !__has_extension(c_atomic) && !__has_extension(cxx_atomic)
+/*
+ * No native support for _Atomic(). Place object in structure to prevent
+ * most forms of direct non-atomic access.
+ */
+#define	_Atomic(T)		struct { T volatile __val; }
+#endif
+
 #if defined(__cplusplus) && __cplusplus >= 201103L
 #define	_Noreturn		[[noreturn]]
 #else

sys/sys/stdatomic.h

@@ -43,10 +43,6 @@
 #error "stdatomic.h does not support your compiler"
 #endif
 
-#if !defined(__CLANG_ATOMICS)
-#define	_Atomic(T)			struct { volatile T __val; }
-#endif
-
 /*
  * 7.17.1 Atomic lock-free macros.
  */