runtime: fix condition to emit gcpacertrace end-of-sweep line

It's the job of the last sweeper to emit the GC pacer trace. The last
sweeper can identify themselves by reducing the count of sweepers, and
also seeing that there's no more sweep work.

Currently this identification is broken, however, because the last
sweeper doesn't check the state they just transitioned sweeping into,
but rather the state they transitioned from (one sweeper, no sweep work
left). By design, it's impossible to transition *out* of this state,
except for another GC to start, but that doesn't take this codepath.

This means lines like

    pacer: sweep done at heap size ...

were missing from the gcpacertrace output for a long time.

This change fixes this problem by having the last sweeper check the
state they just transitioned sweeping to, instead of the state they
transitioned from.

Change-Id: I44bcd32fe2c8ae6ac6c21ba6feb2e7b9e17f60cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/670735


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>