cmd/go: send output of build and install to stderr

"go build" and "go install" were mixing stdout and stderr
from the toolchain, then putting it all on stdout. With this
change, it stays mixed, and is sent to stderr. Because
the toolchain does not create output in a clean compile/install,
sending all output to stderr makese more sense.

Also fix test.bash because of "mktemp: too few X's
in template `testgo'" on Linux.

Fixes issue 4917.

R=golang-dev, rsc, adg
CC=golang-dev
https://codereview.appspot.com/7393073

Committer: Andrew Gerrand <adg@golang.org>