runtime: fix block profile for sync semaphores Part of cl/128670043 that got lost during submit.
runtime: fix block profile for sync semaphores
Part of cl/128670043 that got lost during submit.

TBR=rsc
R=golang-codereviews
CC=golang-codereviews
https://codereview.appspot.com/129570043

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