go/cmd/go: always link external test packages first

go/cmd/go: always link external test packages first

When linking complex packages that use both internal and external tests as well as many dependencies it is critical that the link order be external test package, internal test package, everything else.

This change is a back (forward?) port of the same change that canonical have been maintaining on their fork of the go tool for gccgo. Now that gccgo uses the go tool from upstream, this patch should be applied both to the gofrontend and golang/go repos.

From-SVN: r221800