Revert ABI breakage in libsupc++.

Unfortunately, the ABI was broken upstream for the 4.2 release, which we
imported. We then shipped the broken version for several years and certain
ports (e.g. libobjc2) depend on it, so we're stuck with it for now...

We should revisit this for 10.0, since we're allowed to break the ABI then, but
until then we should keep the ABI we shipped with 8.x and 9.x.

Reviewed by: kan
Approved by: dim (mentor)
MFC after: 1 week