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

As a part of efforts to stabilize Native stdlib.

kotlin-native/backend.native/tests/runtime/workers/worker_exceptions_terminate_hook.kt

@@ -1,4 +1,4 @@
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, ExperimentalStdlibApi::class)
 
 import kotlin.native.concurrent.*
 

kotlin-native/backend.native/tests/runtime/workers/worker_exceptions_terminate_hook_current.kt

@@ -1,4 +1,4 @@
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, ExperimentalStdlibApi::class)
 
 import kotlin.native.concurrent.*