runtime: start new darwin/amd64 threads on correct stack,     then enable stack check.
runtime: start new darwin/amd64 threads on correct stack,
    then enable stack check.

R=r
http://codereview.appspot.com/165100
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go