If the closure pointer gets spilled, we need to spill it with
pointer type to make stack copy and GC happy.

Change-Id: Ic108748e6b9caecd45522141f02c9422567376e3
Reviewed-on: https://go-review.googlesource.com/16363
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>