float.h: C2x NaN and Inf macros

float.h: C2x NaN and Inf macros

C2x adds macros for NaNs and infinities to <float.h>, some of them
previously in <math.h> (and some still in <math.h> as well in C2x as
an obsolescent feature).  Add these macros to GCC's <float.h>
implementation.

This omits the macros for DFP signaling NaNs, leaving those to be
added in a separate patch.  However, it includes the _FloatN /
_FloatNx macros (conditional on __STDC_WANT_IEC_60559_TYPES_EXT__) in
the current draft version of the integration of TS 18661-3 into C2x as
an Annex.

As GCC allows duplicate macro definitions with different expansions in
system headers, it should be OK if <math.h> defines INFINITY or NAN
with a slightly different expansion (e.g. different choice of whether
there is whitespace between tokens); tests are added including
<float.h> and <math.h> in either order.  Because <float.h> uses #undef
on all macros before defining them, even with -Wsystem-headers there
could only ever be issues when <math.h> is included after <float.h>.

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

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

	* ginclude/float.h (INFINITY, NAN, FLT_SNAN, DBL_SNAN, LDBL_SNAN)
	(FLT16_SNAN, FLT32_SNAN, FLT64_SNAN, FLT128_SNAN, FLT32X_SNAN)
	(FLT64X_SNAN, FLT128X_SNAN, DEC_INFINITY, DEC_NAN): New C2x
	macros.
	* doc/sourcebuild.texi (Effective-Target Keywords): Document inff.

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

	* lib/target-supports.exp (check_effective_target_inff): New.
	* gcc.dg/c11-float-4.c, gcc.dg/c11-float-5.c,
	gcc.dg/c11-float-dfp-2.c, gcc.dg/c2x-float-2.c,
	gcc.dg/c2x-float-3.c, gcc.dg/c2x-float-4.c, gcc.dg/c2x-float-5.c,
	gcc.dg/c2x-float-6.c, gcc.dg/c2x-float-7.c, gcc.dg/c2x-float-8.c,
	gcc.dg/c2x-float-9.c, gcc.dg/c2x-float-no-dfp-3.c,
	gcc.dg/c2x-float-no-dfp-4.c, gcc.dg/dfp/c2x-float-dfp-4.c,
	gcc.dg/dfp/c2x-float-dfp-5.c, gcc.dg/dfp/c2x-float-dfp-6.c,
	gcc.dg/torture/float128-nan-floath.c,
	gcc.dg/torture/float128x-nan-floath.c,
	gcc.dg/torture/float16-nan-floath.c,
	gcc.dg/torture/float32-nan-floath.c,
	gcc.dg/torture/float32x-nan-floath.c,
	gcc.dg/torture/float64-nan-floath.c,
	gcc.dg/torture/float64x-nan-floath.c,
	gcc.dg/torture/floatn-nan-floath.h: New tests.