Also re-adjusts build constraints on stubs.

Change-Id: I9c6d8487e17bb1f9e5204e363938a67a7f646c1c
Reviewed-on: https://go-review.googlesource.com/121881
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>