exception handling poll

[John Carr]

> > If that means setjmp exceptions on x86, then we do that.
> 
> Um, are you sure?  setjmp exceptions put extra code in the text section,
> which must be paged in.

No, I'm not sure.  I guess we just can't win if exceptions are enabled
with the current architecture, and that's why I want the binary
incompatibility warning.

> Well, I think we're going to break compatibility for C++ soon.  We
> pretty much have to to support namespaces and templated iostreams.
> And we'll pretty much want to to get more efficient mangling of
> template functions.  Just for fun, try looking at the mangled symbols
> generated for the methods of map<string,string> .

Someone should keep a list of changes that would help gcc but break
compatibility so we can do them all at once.