cmd/go: fix longtest builder after runtime/internal/atomic

cmd/go: fix longtest builder after runtime/internal/atomic

Loosen test to accept staleness in any runtime/internal package.
Fixes longtest builder.

Change-Id: Iffda9c1d1816f76c782aecc4d53c0ea3e6580587
Reviewed-on: https://go-review.googlesource.com/c/go/+/443755
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>