Use -fno-show-column in libstdc++ installed testing.

Use -fno-show-column in libstdc++ installed testing.

<https://gcc.gnu.org/ml/libstdc++/2016-08/msg00006.html> arranged for
libstdc++ tests to use -fno-show-column by default, but only for
build-tree testing.  This patch adds it to the options used for
installed testing as well.

Tested with installed testing for a cross to x86_64-linux-gnu, where
it fixes various test failures.

	Backport from mainline
	2018-10-02  Joseph Myers  <joseph@codesourcery.com>

	* testsuite/lib/libstdc++.exp (libstdc++_init): Use
	-fno-show-column in default cxxflags.

From-SVN: r264955