PATCH: fix dwarf2 handling of signed addresses

[Eric Christopher]

> > My only reservation for this is that we should be sign extending every
> > address no matter what and that this points to a bug in our code
> > elsewhere.
> 
> Are you sure?  Remember, many targets zero extend - Paul's patch looks
> sensible to me.
> 

Of course it's fine, it's in target independent code. I really shouldn't
review the first thing in the morning. :) That said, that it fixes code
in mips still makes me feel wonky.

-eric