This is $GOPATH/pkg/linux_amd64 or similar.  cmd/go already had a grotty calculation
of this and I need to add another one for -buildmode=shared.

Change-Id: Ied28c9b7cce671da8d45920e124a3e0c2501258a
Reviewed-on: https://go-review.googlesource.com/8930Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>