cmd/go: run main package when no files are listed

Fixes 5164.

R=golang-dev, iant, adg
CC=golang-dev
https://codereview.appspot.com/8119049

Committer: Andrew Gerrand <adg@golang.org>
misc/dist: don't ship race-enabled commands

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8350044