[patch] More DWARF-4 support

[Cary Coutant]

> Cary> http://gcc.gnu.org/wiki/ThreadSafetyAnnotation
>
> Cary> How's this? It gives an overview of the whole thread-safety annotation
> Cary> design, and points to the DWARF proposal in Google Docs.
>
> I think that for a GNU extension it would be best for the document to be
> hosted on a GNU site.  That way we have some amount of control over the
> existence and contents of the document.

OK.

I tried translating the document into the wiki markup language, but
gave up (there's too much non-semantic markup, and the wiki doesn't
seem to take any HTML markup), so it looks like the best solution will
be to export the doc as HTML and check it in under wwwdocs/htdocs.

(a) Should this go at the top level or should I create a debug or
dwarf directory?

(b) Should I link to it from anywhere other than the dwarf2.h header?

(c) Do I need to send the HTML file in the form of a patch for review?

-cary