runtime: release Windows thread handle in unminit
runtime: release Windows thread handle in unminit

Fixes issue 8517.

LGTM=dvyukov, alex.brainman
R=golang-codereviews, dvyukov, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/145890044

firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old