cmd/go: final edits for add go list -cgo and -export

These got lost because I violated the cardinal rule of Gerrit
which is never click the Submit button in the web UI.

Change-Id: I8ccdfb5d8691960f244941036d33fb5a5a3f0c9a
Reviewed-on: https://go-review.googlesource.com/117015


Reviewed-by: Russ Cox <rsc@golang.org>