runtime: restore header to first goroutine in Stack

It appears to have been accidentally lost when converting
Stack from C to Go in https://codereview.appspot.com/129510043

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, josharian, khr, remyoudompheng, rsc
https://codereview.appspot.com/136870043