[dev.garbage] runtime:  concurrent mark fixes

Add missing write barrier when initializing state
for newly created goroutine. Add write barrier for
same slot when preempting a goroutine.

Disable write barrier during goroutine death,
because dopanic does pointer writes.

With concurrent mark enabled (not in this CL), all.bash passed once.
The second time, TestGoexitCrash-2 failed.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://codereview.appspot.com/167610043