[build, doc, v3] Support Sun symbol versioning in libstdc++-v3, rev. 2

[Paolo Carlini]

Hi Rainer,
> Ok for mainline?
>   
I didn't study the v3 bits in any detail, but it looks good to me, and
consistent with the plans. Can you confirm that for non-solaris targets
no changes whatsoever are expected?

I say, please wait one more day to be safe, and then the v3 bits are
approved to go in.

Thanks!
Paolo.