cmd/dist: support goos,goarch build tags This is necessary to submit netpoll for linux,386 linux,amd64
cmd/dist: support goos,goarch build tags
This is necessary to submit netpoll for linux,386 linux,amd64

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/7470050

wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old