runtime: fix build

'range hash' makes a copy of the hash array in the stack, creating
a very large stack frame. It's just the right amount that it
uses most but not all of the total stack size. If you have a lot
of environment variables, like the builders, then this is too
much and the g0 stack runs out of space.

TBR=bradfitz
CC=golang-codereviews
https://golang.org/cl/132350043

src/pkg/runtime/iface.go

@@ -427,7 +427,7 @@ func efacethash(e interface{}) uint32 {
 }
 
 func iterate_itabs(fn func(*itab)) {
-	for _, h := range hash {
+	for _, h := range &hash {
 		for ; h != nil; h = h.link {
 			fn(h)
 		}