Define WIDTH macros for C2x.

Define WIDTH macros for C2x.

As part of the integration of TS 18661-1 into C2x, many features
became unconditional features not depending on any feature test macro
being defined.  This patch updates the conditionals on the *_WIDTH
macros in limits.h and stdint.h accordingly so that they are defined
for C2x.  The macro CR_DECIMAL_DIG in float.h does still require
__STDC_WANT_IEC_60559_BFP_EXT__ to be defined, and a test for this is
added.

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

gcc:
	* ginclude/stdint-gcc.h [__STDC_WANT_IEC_60559_BFP_EXT__]: Change
	condition on WIDTH macros to [__STDC_WANT_IEC_60559_BFP_EXT__ ||
	(__STDC_VERSION__ && __STDC_VERSION__ > 201710L)].
	* glimits.h: Likewise.

gcc/testsuite:
	* gcc.dg/cr-decimal-dig-2.c: New test.
	* gcc.dg/limits-width-2.c: New test.  Based on limits-width-1.c.
	* gcc.dg/stdint-width-2.c: New test.  Based on stdint-width-1.c.

From-SVN: r276497