cmd/cover: fix problems with "go test -covermode=atomic sync/atomic"

cmd/cover: fix problems with "go test -covermode=atomic sync/atomic"

This patch fixes an elderly bug with "go test -covermode=atomic
sync/atomic". Change the cover tool to avoid adding an import of
sync/atomic when processing "sync/atomic" itself in atomic mode;
instead make direct calls to AddUint32/StoreUint32. In addition,
change the go command to avoid injecting an artificial import of
"sync/atomic" for sync/atomic itself.

Fixes #57445.

Change-Id: I8c8fbd0bcf26c8a8607d4806046f826296508c74
Reviewed-on: https://go-review.googlesource.com/c/go/+/459335
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>