Make all our package sources use Go 1.17 gofmt format
(adding //go:build lines).
Not strictly necessary but will avoid spurious changes
as files are edited.
Part of //go:build change (#41184).
See https://golang.org/design/draft-gobuild
Change-Id: I30822eb504168b037ed3ec0f7759da1f41251f52
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/294374
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
