runtime: fix TestLFStack on 386

The new(uint64) was moving to the stack, which may not be aligned.

Change-Id: Iad070964202001b52029494d43e299fed980f939
Reviewed-on: https://go-review.googlesource.com/9787Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/runtime/lfstack_test.go

@@ -24,8 +24,12 @@ func toMyNode(node *LFNode) *MyNode {
 	return (*MyNode)(unsafe.Pointer(node))
 }
 
+var global interface{}
+
 func TestLFStack(t *testing.T) {
 	stack := new(uint64)
+	global = stack // force heap allocation
+
 	// Need to keep additional referenfces to nodes, the stack is not all that type-safe.
 	var nodes []*MyNode