[dev.typealias] go/build: add go1.9 build tag

It's earlier than usual but this will help us put the type alias-aware
code into x/tools without breaking clients on go1.6, go1.7,
or (eventually) go1.8.

Change-Id: I43e7ea804922de07d153c7e356cf95e2a11fc592
Reviewed-on: https://go-review.googlesource.com/35050


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>