runtime: fix block profile for sync semaphores And add a test.
runtime: fix block profile for sync semaphores
And add a test.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/128670043

bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc