Update DFP macros in float.h for C2x and TS 18661-2.

Update DFP macros in float.h for C2x and TS 18661-2.

GCC's <float.h> includes various macros for decimal floating-point
types, defined if __STDC_WANT_DEC_FP__, as defined in TS 24732.

TS 18661-2 and C2X make some changes to those macros.  In TS 18661-2,
they are to be defined if __STDC_WANT_IEC_60559_DFP_EXT__, and the
*_SUBNORMAL_MIN macros are renamed to *_TRUE_MIN.  In C2X, the macros
are to be defined whenever DFP is supported, without needing any
feature test macro to be defined.

This patch updates the header accordingly.  There is no existing
predefined macro for whether the target supports DFP; rather than
adding a new macro for that purpose, this patch changes the predefined
macros used in <float.h> so that they are only defined when DFP is
supported and thus can be used as conditionals; this is the same way
predefined macros for _FloatN and _FloatNx types work.  Although
formally TR 24732 and TS 18661-2 say nothing about implementations
lacking DFP support, it seems appropriate to avoid defining the macros
in <float.h>, in the absence of DFP support, even if the respective
feature test macros are defined.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.  Also tested
the c2x-float-no-dfp-* tests for aarch64-linux-gnu to make sure they
do pass in a no-DFP configuration.

gcc:
	* ginclude/float.h [!__DEC32_MANT_DIG__]: Do not define DFP
	macros.
	[__STDC_WANT_IEC_60559_DFP_EXT__ || __STDC_VERSION__ > 201710L]:
	Also define DFP macros for these conditions.
	[!__STDC_WANT_DEC_FP__] (DEC32_SUBNORMAL_MIN, DEC64_SUBNORMAL_MIN,
	DEC128_SUBNORMAL_MIN): Do not define.
	[__STDC_WANT_IEC_60559_DFP_EXT__ || __STDC_VERSION__ > 201710L]
	(DEC32_TRUE_MIN, DEC64_TRUE_MIN, DEC128_TRUE_MIN): New macros.

gcc/c-family:
	* c-cppbuiltin.c (c_cpp_builtins): Do not define macros for DFP
	types if DFP not supported.

gcc/testsuite:
	* gcc.dg/c11-float-dfp-1.c, gcc.dg/c2x-float-no-dfp-1.c,
	gcc.dg/c2x-float-no-dfp-2.c, gcc.dg/dfp/c2x-float-dfp-1.c,
	gcc.dg/dfp/c2x-float-dfp-2.c, gcc.dg/dfp/c2x-float-dfp-3.c,
	gcc.dg/dfp/tr24732-float-dfp-1.c,
	gcc.dg/dfp/ts18661-2-float-dfp-1.c: New tests.

From-SVN: r276854