C++ ABI issues

[Jason Merrill]

There are several C++ ABI incompatibilities between 3.0.x and 3.1.x.  Most
of these will not prevent code built with different compilers from linking
together, but will cause a resulting program to fail in obscure ways.  I'm
not happy with this state of affairs, but I'm not sure what to do about
it.  One answer would be to bump the mangling version for non-ia64 targets,
so that incompatible code will fail to link.

Thoughts?

Jason