[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.*