runtime: convert consistentHeapStats.gen to atomic type
runtime: convert consistentHeapStats.gen to atomic type

For #53821

Change-Id: I9f57b84f6a2c29d750fb20420daef903a9311a83
Reviewed-on: https://go-review.googlesource.com/c/go/+/425781
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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