runtime: convert worldIsStopped to atomic type
runtime: convert worldIsStopped to atomic type

For #53821

Change-Id: I246b65ddb1171d2cab42f98092c64f20ecef392a
Reviewed-on: https://go-review.googlesource.com/c/go/+/425778
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go