runtime: fix GC bitmap corruption
The corruption can occur when GOMAXPROCS
is changed from >1 to 1, since GOMAXPROCS=1
does not imply there is only 1 goroutine running,
other goroutines can still be not parked after
the change.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4873050
3 files changed