Use __f128 to define FLT128_* constants in include/float.h for old GCC.

Use __f128 to define FLT128_* constants in include/float.h for old GCC.

When using compilers before GCC 7, include/float.h provides fallback
definitions of FLT128_* constants.  These definitions use 'Q' constant
suffixes, which works for configurations with _Float128 ABI-distinct
from long double, but not where it has the same ABI as long double.
This patch changes the definitions to use the __f128 macro from
<bits/floatn.h>, so allowing them to work in the non-distinct
_Float128 case (where they are used in building glibc tests, not for
building glibc itself) as well.

Tested (a) with build-many-glibcs.py with GCC 6 (installed stripped
shared libraries unchanged by the patch); (b) with
build-many-glibcs.py with GCC 6 together with the main patch to enable
float128 aliases; (c) for x86_64 with both GCC 6 and GCC 7.

	* include/float.h [!__GNUC_PREREQ (7, 0) && __HAVE_FLOAT128 &&
	__GLIBC_USE (IEC_60559_TYPES_EXT)] (FLT128_MAX): Define using
	__f128.
	[!__GNUC_PREREQ (7, 0) && __HAVE_FLOAT128 && __GLIBC_USE
	(IEC_60559_TYPES_EXT)] (FLT128_EPSILON): Likewise.
	[!__GNUC_PREREQ (7, 0) && __HAVE_FLOAT128 && __GLIBC_USE
	(IEC_60559_TYPES_EXT)] (FLT128_MIN): Likewise.
	[!__GNUC_PREREQ (7, 0) && __HAVE_FLOAT128 && __GLIBC_USE
	(IEC_60559_TYPES_EXT)] (FLT128_TRUE_MIN): Likewise.