[v3, testsuite] Fix merging default libstdc++.log

[Mike Stump]

On Apr 16, 2012, at 8:03 AM, Rainer Orth wrote:
> I've long noticed that libstdc++.log (unlike libstdc++.sum) doesn't
> contain log entries for tests run from abi.exp, but hadn't looked
> closer, getting used to check libstdc++.log.sep instead which contained
> everything I expected.

> ok for mainline?

Ok.  Would have been nice to see the before and after log file...