C++ ABI Issues

[Joe Buck]

> Yes.  Furthermore, libstdc++ has an ABI versioning both in the source
> code and in the built binary.  I much prefer we use those menchanisms.

We may not need them: I believe that libstdc++ is not affected at all.  It
appears that the only uses of virtual baseclasses are for basic_istream
and basic_ostream, and the last field of their base class is a pointer,
indicating that there will be no padding space on ILP32 or LP64 systems.
Mark's patch should confirm this.  But if I'm right, libstdc++ needs no
versioning at all.