[v3] libstdc++/17922

[Benjamin Kosnik]

>... and sorry for not not having replied sooner to your hint in the 
>audit trail: I was sad because of the _S_ios_openmode_end & co bits, 
>that probably we can't really fix within the current ABI: for v7 maybe 
>we should bite the bullet and renounce to the enums?!?

I think this patch is safe for mainline and gcc-3_4-branch. Adding
values is not going to cause any problems.

Is there something I just don't see at the moment?

-benjamin