TS 18661-1 defines a macro CR_DECIMAL_DIG in <float.h>, for the number
of decimal digits for which conversions between decimal character
strings and (IEEE) binary formats, in both directions, are correctly
rounded.  This patch implements support for this macro in GCC's
<float.h>.

The definition __UINTMAX_MAX__ is the right one for GCC's conversions
of floating constants, since I made those use MPFR to make them
correctly rounding.  The macro also covers standard library functions
such as strtod and printf.  The definition is also correct for current
glibc.  If any targets' libcs support correct rounding in a way that
conforms to TS 18661-1 with a smaller value of CR_DECIMAL_DIG, making
<float.h> reflect that could not be done in isolation without changes
to the interpretation of floating constants as well, since a smaller
CR_DECIMAL_DIG requires double rounding of floating constants (first
to CR_DECIMAL_DIG decimal digits, then to the desired binary format).

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

gcc:
	* ginclude/float.h [__STDC_WANT_IEC_60559_BFP_EXT__]
	(CR_DECIMAL_DIG): New macro.

gcc/testsuite:
	* gcc.dg/cr-decimal-dig-1.c: New test.

From-SVN: r240249