The only reason these flushes are still necessary at all is that
gcmarknewobject doesn't flush its gcWork stats like it's supposed to.
By changing gcmarknewobject to follow the standard protocol, the
flushes become completely unnecessary because mark 2 ensures caches
are flushed (and stay flushed) before we ever enter mark termination.

In the garbage benchmark, this takes roughly 50 µs, which is
surprisingly long for doing nothing. We still double-check after
draining that they are in fact empty.

Change-Id: Ia1c7cf98a53f72baa513792eb33eca6a0b4a7128
Reviewed-on: https://go-review.googlesource.com/31134
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>