Fix powerpc64le build of nan-sign tests (bug 23303).
My recent nan-sign tests fail to build for powerpc64le with GCC 8
because of the special compile / link options needed there for any
test using _Float128. This patch arranges for these tests to be
handled on powerpc64le similarly to other such tests.
Tested with build-many-glibcs.py for powerpc64le.
[BZ #23303]
* sysdeps/powerpc/powerpc64/le/Makefile
(CFLAGS-tst-strtod-nan-sign.c): Add -mfloat128.
(CFLAGS-tst-wcstod-nan-sign.c): Likewise.
(gnulib-tests): Also add $(f128-loader-link) for
tst-strtod-nan-sign abd tst-wcstod-nan-sign.