Move float128 inlines from sysdeps/generic/math_private.h to include/math.h.

Continuing the cleanup of math_private.h, with a view to it becoming
the header for the APIs defined therein and not also a header with
inline variants of math.h APIs, this patch moves inline definitions of
__isinff128 and fabsf128 to include/math.h, so that any users of
math.h in glibc automatically get the optimized functions rather than
quietly missing them if they do not also include math_private.h.

Tested for x86_64 and x86, and with build-many-glibcs.py with GCC 6.
There are changes to installed stripped libc.so on configurations with
distinct _Float128, because of __printf_fp_l code that now gets the
__isinff128 inline where previously it called the out-of-line
function because of the lack of a math_private.h call.  It seems
appropriate that this code does get the inline (as it would
automatically with GCC 7 and later when the built-in function is used)
rather than being the only place in glibc that does not.

	* sysdeps/generic/math_private.h
	[__HAVE_DISTINCT_FLOAT128 && !__GNUC_PREREQ (7, 0)] (__isinff128):
	Move this inline function ....
	[__HAVE_DISTINCT_FLOAT128] (fabsf128): And this one ....
	* include/math.h [!_ISOMAC]: To here....