cmd/go: ignore coverpkg match on sync/atomic in atomic coverage mode

Otherwise we get into a dependency loop as we try to apply coverage
analysis to sync/atomic when the coverage analysis itself requires
sync/atomic.

Fixes #23694

Change-Id: I3a74ef3881ec5c6197ed348acc7f9e175417f6c7
Reviewed-on: https://go-review.googlesource.com/91875


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>