reflect: use a bigger object when we need a finalizer to run

If an object is allocated as part of a tinyalloc, then other live
objects in the same tinyalloc chunk keep the finalizer from being run,
even if the object that has the finalizer is dead.

Make sure the object we're setting the finalizer on is big enough
to not trigger tinyalloc allocation.

Fixes #26857
Update #21717

Change-Id: I56ad8679426283237ebff20a0da6c9cf64eb1c27
Reviewed-on: https://go-review.googlesource.com/128475
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/reflect/all_test.go

@@ -1693,9 +1693,9 @@ func TestCallReturnsEmpty(t *testing.T) {
 	// nonzero-sized frame and zero-sized return value.
 	runtime.GC()
 	var finalized uint32
-	f := func() (emptyStruct, *int) {
-		i := new(int)
-		runtime.SetFinalizer(i, func(*int) { atomic.StoreUint32(&finalized, 1) })
+	f := func() (emptyStruct, *[2]int64) {
+		i := new([2]int64) // big enough to not be tinyalloc'd, so finalizer always runs when i dies
+		runtime.SetFinalizer(i, func(*[2]int64) { atomic.StoreUint32(&finalized, 1) })
 		return emptyStruct{}, i
 	}
 	v := ValueOf(f).Call(nil)[0] // out[0] should not alias out[1]'s memory, so the finalizer should run.