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

[Jonathan Wakely]

On 02/10/18 00:58 +0000, Joseph Myers wrote:
><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.

Great, thanks.

This is OK for trunk, and I have no objections to also changing it for
the branches if you want it there.