remove gccgo restriction with respect to blank identifiers

R=iant
CC=go-dev
http://go/go-review/1016032

doc/go_spec.html

@@ -4582,6 +4582,5 @@ The following minimal alignment properties are guaranteed:
 <h2 id="Implementation_differences"><span class="alert">Implementation differences - TODO</span></h2>
 <ul>
 	<li><span class="alert">Implementation does not honor the restriction on goto statements and targets (no intervening declarations).</span></li>
-	<li><span class="alert">Gccgo does not implement the blank identifier.</span></li>
 	<li><span class="alert">Method expressions are not implemented.</span></li>
 </ul>