runtime: apply KindMask before comparison
runtime: apply KindMask before comparison

Fixes windows build.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/127510043

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