float.h: C2x *_IS_IEC_60559 macros

float.h: C2x *_IS_IEC_60559 macros

C2x adds float.h macros that say whether float, double and long double
match an IEC 60559 (IEEE 754) format and operations.  Add these
macros to GCC's float.h.

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

gcc/c-family/
2020-11-17  Joseph Myers  <joseph@codesourcery.com>

	* c-cppbuiltin.c (builtin_define_float_constants): Define
	"*_IS_IEC_60559__" macros.

gcc/
2020-11-17  Joseph Myers  <joseph@codesourcery.com>

	* ginclude/float.h [__STDC_VERSION__ > 201710L] (FLT_IS_IEC_60559,
	DBL_IS_IEC_60559, LDBL_IS_IEC_60559): New macros.

gcc/testsuite/
2020-11-17  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c11-float-6.c, gcc.dg/c2x-float-10.c: New tests.