With the hybrid barrier, unless we're doing a STW GC or hit a very
rare race (~once per all.bash) that can start mark termination before
all of the work is drained, we don't need to drain the work queue at
all. Even draining an empty work queue is rather expensive since we
have to enter the getfull() barrier, so it's worth avoiding this.

Conveniently, it's quite easy to detect whether or not we actually
need the getufull() barrier: since the world is stopped when we enter
mark termination, everything must have flushed its work to the work
queue, so we can just check the queue. If the queue is empty and we
haven't queued up any jobs that may create more work (which should
always be the case with the hybrid barrier), we can simply have all GC
workers perform non-blocking drains.

Also conveniently, this solution is quite safe. If we do somehow screw
something up and there's work on the work queue, some worker will
still process it, it just may not happen in parallel.

This is not the "right" solution, but it's simple, expedient,
low-risk, and maintains compatibility with debug.gcrescanstacks. When
we remove the gcrescanstacks fallback in Go 1.9, we should also fix
the race that starts mark termination early, and then we can eliminate
work draining from mark termination.

Updates #17503.

Change-Id: I7b3cd5de6a248ab29d78c2b42aed8b7443641361
Reviewed-on: https://go-review.googlesource.com/32186Reviewed-by: Rick Hudson <rlh@golang.org>