runtime: don't report non-blocked goroutines as "(durable)" in stacks

runtime: don't report non-blocked goroutines as "(durable)" in stacks

Only append the " (durable)" suffix to a goroutine's status
when the goroutine is waiting.

Avoids reporting a goroutine as "runnable (durable)".

Change-Id: Id679692345afab6e63362ca3eeff16808367e50f
Reviewed-on: https://go-review.googlesource.com/c/go/+/705995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>