doc: clarify that gccgo is a GCC front-end

R=golang-dev, r, iant, iant
CC=golang-dev
https://golang.org/cl/6454072

doc/go_faq.html

@@ -1407,7 +1407,7 @@ test cases. The standard Go library is full of illustrative examples, such as in
 What compiler technology is used to build the compilers?</h3>
 
 <p>
-<code>Gccgo</code> has a C++ front-end with a recursive descent parser coupled to the
+<code>Gccgo</code> has a front end written in C++, with a recursive descent parser coupled to the
 standard GCC back end. <code>Gc</code> is written in C using
 <code>yacc</code>/<code>bison</code> for the parser.
 Although it's a new program, it fits in the Plan 9 C compiler suite