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

dsriram/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang