[win32] Fix couple of bugs in signal unwinder

[Eric Botcazou]

> Everything is obscured on Cygwin target by other problems, I'll have to
> retest on MinGW. There will now be an interlude...

Yes, our Windows compiler is MinGW-based.  But it just occured to me that the 
FSF Ada compiler is still configured for SJLJ exceptions:

   ZCX_By_Default            : constant Boolean := False;

in system-mingw.ads so my patch is actually a no-op for Ada. *sigh*

-- 
Eric Botcazou