Copying return values to heap-escaped result parameters requires write
barriers.

Fixes #13587.

Change-Id: Ifa04ff7fa4adcc6393acdd82e527beb8f2a00a8b
Reviewed-on: https://go-review.googlesource.com/17762Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>