Fix narrowing function tests build for powerpc64le.

Testing narrowing functions with build-many-glibcs.py showed up a
further testsuite fix needed to enable building such functions for
powerpc64le: tests test-<narrower-type>-float128-<function> (and
likewise for float64x) needed the same special handling for
powerpc64le as test-float128-* and test-float64x-*.  This patch adds
that special handling.

Tested with build-many-glibcs.py for powerpc64le in conjunction with
the main patch adding narrowing add functions.

	* sysdeps/powerpc/powerpc64le/Makefile [$(subdir) = math]
	(f128-pairs): New variable.
	[$(subdir) = math] ($(foreach suf,$(all-object-suffixes),$(foreach
	pair,$(f128-pairs),$(objpfx)test-$(pair)%$(suf)))): Add -mfloat128
	to CFLAGS.
	[$(subdir) = math] ($(foreach pair,$(f128-pairs),test-$(pair)%)):
	Also make tests add $(f128-loader-link) to gnulib-tests.