ginclude: C2x header version macros

C2x adds __STDC_VERSION_*_H__ macros to individual headers with
interface changes compared to C17.  All the new header features in
headers provided by GCC have now been implemented, so define those
macros to the value given in the current working draft.

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

gcc/
	* ginclude/float.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_FLOAT_H__): New macro.
	* ginclude/stdarg.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_STDARG_H__): New macro.
	* ginclude/stdatomic.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_STDATOMIC_H__): New macro.
	* ginclude/stddef.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_STDDEF_H__): New macro.
	* ginclude/stdint-gcc.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_STDINT_H__): New macro.
	* glimits.h [__STDC_VERSION__ > 201710L]
	(__STDC_VERSION_LIMITS_H__): New macro.

gcc/testsuite/
	* gcc.dg/c11-float-8.c, gcc.dg/c11-limits-1.c,
	gcc.dg/c11-stdarg-4.c, gcc.dg/c11-stdatomic-3.c,
	gcc.dg/c11-stddef-1.c, gcc.dg/c11-stdint-1.c,
	gcc.dg/c2x-float-13.c, gcc.dg/c2x-limits-1.c,
	gcc.dg/c2x-stdarg-5.c, gcc.dg/c2x-stdatomic-1.c,
	gcc.dg/c2x-stddef-1.c, gcc.dg/c2x-stdint-1.c: New tests.