Do not declare _Float128 support for powerpc64le -mlong-double-64 (bug 22402).

Do not declare _Float128 support for powerpc64le -mlong-double-64 (bug 22402).

The powerpc bits/floatn.h declares _Float128 support to be present
when the compiler supports it for powerpc64le.  However, in the case
where -mlong-double-64 is used, __MATH_TG does not actually support
_Float128; it only supports _Float128 in the distinct-long-double
case.

This shows up as a build failure when building glibc mainline with GCC
mainline, given the recently added sanity check in math.h for
configurations supported by __MATH_TG, as the compat code for
-mlong-double-64 fails to build.  However, the bug was logically
present before that change (including in 2.26), just less visible.

This patch fixes the build failure by declaring _Float128 to be
unsupported in that case.  (Of course this can't actually stop users
calling the type-generic macros with _Float128 arguments with
-mlong-double-64, just as they could be called with other unsupported
types on other platforms, but perhaps makes it less likely by making
all the type-specific _Float128 interfaces invisible in that case.)

Tested compilation for powerpc64le with build-many-glibcs.py.

	[BZ #22402]
	* sysdeps/powerpc/bits/floatn.h: Include <bits/long-double.h>.
	[__NO_LONG_DOUBLE_MATH] (__HAVE_FLOAT128): Define to 0.