Adjust gccgo license references.

gccgo will be distributed under the same license as the rest
of the Go code.

R=r, rsc
http://go/go-review/1024019

doc/gccgo_contribute.html

@@ -14,11 +14,8 @@ gccgo</a>.
 <h2>Legal Prerequisites</h2>
 
 <p>
-The <code>gccgo</code> frontend is part of the GCC compiler, and it
-follows the <a href="http://gcc.gnu.org/contribute.html">GCC
-contribution rules</a>.  In particular, for substantial
-patches&mdash;more than 10 lines&mdash;you must have a copyright
-assignment with the Free Software Foundation.
+You must follow the <a href="contribute.html#copyright">Go copyright
+rules.</a>
 </p>
 
 <h2>Code</h2>

doc/gccgo_install.html

@@ -3,9 +3,9 @@
 <p>
 This document explains how to use <code>gccgo</code>, a compiler for
 the Go language.  The <code>gccgo</code> compiler is a new frontend
-for <code>gcc</code>, the widely used GNU compiler.
-Like <code>gcc</code> itself, <code>gccgo</code> is free software
-distributed under
+for <code>gcc</code>, the widely used GNU compiler.  Although the
+frontend itself is under a BSD-style license, <code>gccgo</code> is
+normally used as part of <code>gcc</code> and is then covered by
 the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public
 License</a>.
 </p>