runtime: do not garbage collect windows callbacks
runtime: do not garbage collect windows callbacks

Fixes issue 1883.
Fixes issue 1702.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4532103
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go