[release-branch.go1.8] cmd/link: use external linking for PIE by default

Now `go test -buildmode=pie std -short` passes on linux/amd64.

Updates #18968

Change-Id: Ide21877713e00edc64c1700c950016d6bff8de0e
Reviewed-on: https://go-review.googlesource.com/36417


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/36421


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>