sweepone returns ^uintptr(0) when there are no more spans to *start*
sweeping, but there may be spans being swept concurrently at the time
and there's currently no efficient way to tell when the sweeper is
done sweeping all the spans.

We'll need this for concurrent runtime.GC(), so add a count of the
number of active sweepone calls to make it possible to block until
sweeping is truly done.

This is also useful for more accurately printing the gcpacertrace,
since that should be printed after all of the sweeping stats are in
(currently we can print it slightly too early).

For #18216.

Change-Id: I06e6240c9e7b40aca6fd7b788bb6962107c10a0f
Reviewed-on: https://go-review.googlesource.com/37716
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>