runtime: add more tests for LockOSThread()
Just test some additional paths through the scheduler.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7331044
cmd/gc: remove node printing in redeclare errors

I suspect this is some debugging which got through the submission process.

Fixes issue 4789.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7310079