[K/N] Mark WeakReference with ExperimentalNativeApi

As a part of efforts to stabilize Native stdlib #KT-55765.

kotlin-native/backend.native/tests/runtime/memory/cycles1.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.cycles1
 
 import kotlin.test.*

kotlin-native/backend.native/tests/runtime/memory/weak0.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.weak0
 
 import kotlin.test.*

kotlin-native/backend.native/tests/runtime/memory/weak1.kt

@@ -3,6 +3,8 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(kotlin.experimental.ExperimentalNativeApi::class)
+
 package runtime.memory.weak1
 
 import kotlin.test.*