runtime: restore header to first goroutine in Stack

It appears to have been accidentally lost when converting
Stack from C to Go in https://golang.org/cl/129510043

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, josharian, khr, remyoudompheng, rsc
https://golang.org/cl/136870043

src/pkg/runtime/mprof.go

@@ -164,6 +164,7 @@ func Stack(buf []byte, all bool) int {
 	if len(buf) > 0 {
 		gp.writebuf = &buf[0]
 		gp.writenbuf = int32(len(buf))
+		goroutineheader(gp)
 		traceback(pc, sp, 0, gp)
 		if all {
 			tracebackothers(gp)

src/pkg/runtime/stubs.go

@@ -216,6 +216,7 @@ func gonotetsleepg(n *note, t int64) {
 
 func exitsyscall()
 
+func goroutineheader(gp *g)
 func traceback(pc, sp, lr uintptr, gp *g)
 func tracebackothers(gp *g)