libstdc++/5625: exception unwinding creates invalid pointer o n mips

[Ed Maste]

> > If _Unwind_Ptr were signed, the right thing should happen.  (iirc; i
> > seem to recall having discussed this with Ed previously [and him
> > having confirmed that result, but I may be mistaken].)

That is correct.  

When I discovered the issue I wanted to change as little as possible 
to isolate it so I left the _Unwind_Ptr typedef alone and rebuilt just
eh_personality.

-ed