This moves concurrent sweep termination from the coordinator to the
off->mark transition. This allows it to be performed by all Gs
attempting to start the GC.

Updates #11970.

Change-Id: I24428e8599a759398c2ef7ec996ba755a448f947
Reviewed-on: https://go-review.googlesource.com/16356Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>