Currently checkmarks mode fails to rescan stacks because it sees the
leftover state bits indicating that the stacks haven't changed since
the last scan. As a result, it won't detect lost marks caused by
failing to scan stacks correctly during regular garbage collection.

Fix this by marking all stacks dirty before performing the checkmark
phase.

Change-Id: I1f06882bb8b20257120a4b8e7f95bb3ffc263895
Reviewed-on: https://go-review.googlesource.com/10794Reviewed-by: Russ Cox <rsc@golang.org>