[wwwdocs] Add gcc.html

[Gerald Pfeifer]

To meet standards (and server configuration) on www.gnu.org where
our web pages are hosted as well, I added a trivial file gcc.html
to our web pages:

<meta http-equiv="refresh" content="0; url=index.html">

This redirects to our index page and should _not_ be referenced
anywhere within our control, be it from our own web pages or
externally, it is solely there for the purpose of www.gnu.org.

Gerald