doc: clarify that gccgo is a GCC front-end
doc: clarify that gccgo is a GCC front-end

R=golang-dev, r, iant, iant
CC=golang-dev
https://codereview.appspot.com/6454072

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang