Add a space before build tag comments so it corresponds to the format
documented at https://golang.org/pkg/go/build/.

Change-Id: I8349d0343597e304b97fb5479847231ed8945b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/193237
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>