Not a complete fix for issue 3342, but fixes the trivial case.
There may still be a race in the instants before and after
a scavenger-induced garbage collection.
Intended to be "obviously safe": a call to runtime·gosched
before main.main is no different than a call to runtime.Gosched
at the beginning of main.main, and it is (or had better be)
safe to call runtime.Gosched at any point during main.
Update #3342.
R=iant
CC=golang-dev
https://golang.org/cl/5919052
Breaks closure test when GOMAXPROCS=2 or more.
««« original CL description
runtime: restore deadlock detection in the simplest case.
Fixes#3342.
R=iant, r, dave, rsc
CC=golang-dev, remy
https://golang.org/cl/5844051
»»»
R=rsc
CC=golang-dev
https://golang.org/cl/5924045
