[K/N] Mark UnhandledExceptionHook-related API as experimental

As a part of efforts to stabilize Native stdlib.

kotlin-native/backend.native/tests/produce_dynamic/simple/hello.kt

@@ -4,7 +4,7 @@
  */
 
 // FILE: hello.kt
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, ExperimentalStdlibApi::class)
 
 import kotlinx.cinterop.*
 

kotlin-native/backend.native/tests/produce_dynamic/unhandledException/unhandled.kt

@@ -2,7 +2,7 @@
  * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, ExperimentalStdlibApi::class)
 
 import kotlin.native.concurrent.*
 import kotlin.native.internal.*