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.