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.

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

From-SVN: r264784

libstdc++-v3/ChangeLog

+2018-10-02  Joseph Myers  <joseph@codesourcery.com>
+
+	* testsuite/lib/libstdc++.exp (libstdc++_init): Use
+	-fno-show-column in default cxxflags.
+
 2018-10-01  Bernhard Reutner-Fischer  <aldot@gcc.gnu.org>
 
 	* config/os/uclibc/os_defines.h (__NO_STRING_INLINES): Delete.

libstdc++-v3/testsuite/lib/libstdc++.exp

@@ -239,7 +239,7 @@ proc libstdc++_init { testfile } {
 
     # Default settings.
     set cxx [transform "g++"]
-    set cxxflags "-fmessage-length=0"
+    set cxxflags "-fmessage-length=0 -fno-show-column"
     set cxxpchflags ""
     set cxxvtvflags ""
     set cxxldflags ""