sync/atomic: use consistent first-store-in-progress marker

sync/atomic: use consistent first-store-in-progress marker

We need to use the same marker everywhere. My CL to rename the
marker (CL 241661) and the CL to add more uses of the marker
under the old name (CL 241678) weren't coordinated with each other.

Fixes #52612

Change-Id: I97023c0769e518491924ef457fe03bf64a2cefa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/403094
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>