runtime: free stacks of Gdead goroutines at GC time

We could probably free the G structures as well, but
for the allg list.  Leaving that for another day.

Fixes issue 8287

LGTM=rsc
R=golang-codereviews, dvyukov, khr, rsc
CC=golang-codereviews
https://codereview.appspot.com/145010043