Allow macros prefixed with FLT128 in include/float.h

Allow macros prefixed with FLT128 in include/float.h

TS 18661-3 specifies that macros prefixed with FLTN_ can be defined in
float.h, only if __STDC_WANT_IEC_60559_TYPES_EXT__ is defined as a macro
before the inclusion of float.h.  Since GCC 7.0, these macros are provided
under this condition, however, for older versions of GCC, these macros are
not provided at all.  This patch allows the definitions of such macros in
include/float.h for older compilers, if the condition above is met, and
even if _ISOMAC is defined.

	* include/float.h: Allow the definition of macros prefixed with
	FLT128 even if _ISOMAC is defined, but provided that
	__STDC_WANT_IEC_60559_TYPES_EXT__ is defined as a macro.