cmd/dist: use GOHOSTARCH/GOHOSTOS instead of GOOS/GOARCH for host libraries and binaries
cmd/dist: use GOHOSTARCH/GOHOSTOS instead of GOOS/GOARCH for host libraries and binaries

Fixes issue 6559

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/81330045

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