cmd/vet: check sync.* types' copying

cmd/vet: check sync.* types' copying

Embed noLock struct into the following types, so `go vet -copylocks` catches
their copying additionally to types containing sync.Mutex:
  - sync.Cond
  - sync.WaitGroup
  - sync.Pool
  - atomic.Value

Fixes #14582

Change-Id: Icb543ef5ad10524ad239a15eec8a9b334b0e0660
Reviewed-on: https://go-review.googlesource.com/22015
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>