cmd/go: set the "generate" build tag in go generate, per design doc

And use it in two internal windows packages, so they don't show up in
"go list std" or binary releases on non-Windows platforms.

Fixes #31920

Change-Id: Iaa292b6015c9d7310dd677c9e296006440ba5e27
Reviewed-on: https://go-review.googlesource.com/c/go/+/175983


Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>