In general, finishsweep_m must block until any spans that are
concurrently being swept have been swept. It accomplishes this by
looping over all spans, which, as in the previous commit, takes
~1ms/heap GB. Unfortunately, we do this during the STW sweep
termination phase, so multi-gigabyte heaps can push our STW time past
10ms.

However, there's no need to do this wait if the world is stopped
because, in effect, stopping the world already had to wait for
anything that was sweeping (and if it didn't, the wait in
finishsweep_m would deadlock). Hence, we can simply skip this loop if
the world is stopped, such as during sweep termination. In fact,
currently all calls to finishsweep_m are STW, but this hasn't always
been the case and may not be the case in the future, so we keep the
logic around.

For 24GB heaps, this reduces max pause time by 75% relative to tip and
by 90% relative to Go 1.5. Notably, all pauses are now well under
10ms. Here are the results for the garbage benchmark:

               ------------- max pause ------------
Heap   Procs   after change   before change   1.5.1
24GB     12        3.8ms          16ms         37ms
24GB      4        3.7ms          16ms         37ms
 4GB      4        3.7ms           3ms        6.9ms

In the 4GB/4P case, it seems the "before change" run got lucky: the
max went up, but the 99%ile pause time went down from 3ms to 2.04ms.

Change-Id: Ica22189559f231d408ef2815019c9dbb5f38bf31
Reviewed-on: https://go-review.googlesource.com/15071Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>