cmd/go: fix -covermode=atomic use of sync/atomic in -coverpkg matches
If we're using -covermode=atomic with -coverpkg, to add coverage
to more than just the package being tested, then we need to make sure
to make sync/atomic available to the compiler for every package
being recompiled for coverage.
Fixes #22728.
Change-Id: I27f88f6a62e37d4a7455554cd03c8ca2b21f81a4
Reviewed-on: https://go-review.googlesource.com/81497
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>