[3.3, 3.4, HEAD] Include baseline used for libstdc++-v3 abi check in test summary

[Paolo Carlini]

Mark Mitchell wrote:

>> Any reason not to checkin?
>
> Not from me, but this is up to the V3 folks.

... too easy ;)

Seriously, the change actually belongs to contrib, *not* a directory 
under libstdc++-v3,
therefore I (we, in my opinion) can't really approve it.

Paolo.