Add support for testing __STDC_WANT_IEC_60559_TYPES_EXT__

This macro is defined by TS 18661-3 for supporting the _FloatN and
_FloatNx types, as well as the functions suffixed with fN.

	* bits/libc-header-start.h:
	(__GLIBC_USE_IEC_60559_TYPES_EXT): New macro.
	* include/features.h: Describe __STDC_WANT_IEC_60559_TYPES_EXT__.
	* manual/creature.texi: Likewise.