[patch] More DWARF-4 support

[Cary Coutant]

> I've added the DWARF extensions document as an HTML page at
> wwwdocs/htdocs/projects/thread-annotations.html, which I will send as
> a separate patch, then I'll update the ThreadSafetyAnnotation wiki
> page to point to that page instead of Google Docs.

Update... Cleaning up the HTML proved to be just as painful as marking
it up for the wiki, so I went ahead and put the document on the gcc
wiki:

   http://gcc.gnu.org/wiki/ThreadSafetyAnnotationsInDWARF

The patched dwarf2.h still points to the ThreadSafetyAnnotation page,
which now refers to the new DWARF page. I think that's still the
better starting point, so the patch is unchanged by this.

-cary