runtime: add fast paths to non-blocking channel operations
benchmark                      old ns/op     new ns/op     delta
BenchmarkChanNonblocking       27.8          7.80          -71.94%
BenchmarkChanNonblocking-2     79.1          3.94          -95.02%
BenchmarkChanNonblocking-4     71.2          2.04          -97.13%

LGTM=rsc
R=golang-codereviews, rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/110580043
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