cmd/cgo: document command line options
cmd/cgo: document command line options

Also remove incorrect statement that cgo does not support
gccgo.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13511043

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