float.h: C2x decimal signaling NaN macros

float.h: C2x decimal signaling NaN macros

C2x adds macros for decimal floating-point signaling NaNs to
<float.h>.  Add these macros to GCC's <float.h> implementation.

Note that the current C2x draft has these under incorrect names
D32_SNAN, D64_SNAN, D128_SNAN.  The intent was to change the naming
convention to be consistent with other <float.h> macros when they were
moved to <float.h>, so DEC32_SNAN, DEC64_SNAN, DEC128_NAN, which this
patch uses (as does the current draft integration of TS 18661-3 as an
Annex to C2x, for its _Decimal* and _Decimal*x types).

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

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

	* ginclude/float.h (DEC32_SNAN, DEC64_SNAN, DEC128_SNAN): New C2x
	macros.

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

	* gcc.dg/dfp/c2x-float-dfp-7.c, gcc.dg/dfp/c2x-float-dfp-8.c: New
	tests.
	* gcc.dg/c2x-float-no-dfp-3.c: Also check that DEC32_SNAN,
	DEC64_SNAN and DEC128_SNAN are not defined.