stdatomic.h: Do not define ATOMIC_VAR_INIT for C2x

The <stdatomic.h> macro ATOMIC_VAR_INIT, previously declared obsolete,
is removed completely in C2x; disable it for C2x in GCC's
implementation.  (Although ATOMIC_* are reserved names for this
header, disabling the macro for C2x still seems appropriate.)

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/
	* ginclude/stdatomic.h [defined __STDC_VERSION__ &&
	__STDC_VERSION__ > 201710L] (ATOMIC_VAR_INIT): Do not define.

gcc/testsuite/
	* gcc.dg/atomic/c2x-stdatomic-var-init-1.c: New test.

gcc/ginclude/stdatomic.h

@@ -79,7 +79,9 @@ typedef _Atomic __INTMAX_TYPE__ atomic_intmax_t;
 typedef _Atomic __UINTMAX_TYPE__ atomic_uintmax_t;        
 
 
+#if !(defined __STDC_VERSION__ && __STDC_VERSION__ > 201710L)
 #define ATOMIC_VAR_INIT(VALUE)	(VALUE)
+#endif
 
 /* Initialize an atomic object pointed to by PTR with VAL.  */
 #define atomic_init(PTR, VAL)                           \

gcc/testsuite/gcc.dg/atomic/c2x-stdatomic-var-init-1.c

+/* Test ATOMIC_VAR_INIT not in C2x.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+#include <stdatomic.h>
+
+#ifdef ATOMIC_VAR_INIT
+#error "ATOMIC_VAR_INIT defined"
+#endif