[dev.garbage] runtime: fix TestLFStack on 386

LGTM=rlh
R=rlh, dvyukov
CC=golang-codereviews
https://golang.org/cl/157430044

src/runtime/export_test.go

@@ -26,7 +26,7 @@ var Exitsyscall = exitsyscall
 var LockedOSThread = lockedOSThread
 
 type LFNode struct {
-	Next    *LFNode
+	Next    uint64
 	Pushcnt uintptr
 }
 

src/runtime/lfstack_test.go

@@ -121,7 +121,7 @@ func TestLFStackStress(t *testing.T) {
 			}
 			cnt++
 			sum2 += node.data
-			node.Next = nil
+			node.Next = 0
 		}
 	}
 	if cnt != K {

src/runtime/runtime.h

@@ -571,6 +571,7 @@ enum {
 #endif
 
 // Lock-free stack node.
+// Also known to export_test.go.
 struct LFNode
 {
 	uint64	next;