cmd/go: add go generate -skip flag

cmd/go: add go generate -skip flag

Following proposal discussion in #38687, add go generate -skip
to allow easier skipping of specific //go:generate directives.

Fixes #38687.

Change-Id: Ied5b4042965dd6a2b93c1c517045fccae2d95c3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/421440
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>