gccgo_install.html: the 4.9 release series supports Go 1.2

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/163760043

doc/gccgo_install.html

@@ -42,6 +42,10 @@ identical to Go 1.1.  The GCC 4.8.2 release includes a complete Go
 1.1.2 implementation.
 </p>
 
+<p>
+The GCC 4.9 releases include a complete Go 1.2 implementation.
+</p>
+
 <h2 id="Source_code">Source code</h2>
 
 <p>