Handle more _FloatN, _FloatNx types in include/float.h.

Continuing the preparation for additional _FloatN / _FloatNx type
support, this patch adds support in include/float.h.  This header
defines macros for _Float128 properties when using compilers before
GCC 7 that lack those macros in <float.h>.  For testing _Float32 /
_Float64 / _Float32x / _Float64x functions with older compilers, such
macros need to be defined for those types as well; for the older
compilers, those types will always be typedefs for another type, so
the definitions can be in terms of the macros for that other type.

Tested for x86_64.

	* include/float.h
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MANT_DIG): New macro.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_DECIMAL_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MIN_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MIN_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MAX_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MAX_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MAX): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_EPSILON): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32] (FLT32_TRUE_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MANT_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_DECIMAL_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MIN_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MIN_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MAX_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MAX_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MAX): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_EPSILON): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64] (FLT64_TRUE_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MANT_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_DECIMAL_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MIN_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MIN_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MAX_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MAX_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MAX): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_EPSILON): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT32X] (FLT32X_TRUE_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MANT_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_DECIMAL_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_DIG): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MIN_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MIN_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MAX_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MAX_10_EXP): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MAX): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_EPSILON): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __GLIBC_USE (IEC_60559_TYPES_EXT)
	&& __HAVE_FLOAT64X] (FLT64X_TRUE_MIN): Likewise.