[K/N] Move kotlin.native.internal.GC into kotlin.native.runtime

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

kotlin-native/backend.native/tests/runtime/basic/cleaner_basic.kt

@@ -2,7 +2,7 @@
  * Copyright 2010-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
-@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class)
+@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class, kotlin.native.runtime.NativeRuntimeApi::class)
 
 package runtime.basic.cleaner_basic
 
@@ -13,6 +13,7 @@ import kotlin.native.concurrent.*
 import kotlin.native.ref.WeakReference
 import kotlin.native.ref.Cleaner
 import kotlin.native.ref.createCleaner
+import kotlin.native.runtime.GC
 
 class AtomicBoolean(initialValue: Boolean) {
     private val impl = AtomicInt(if (initialValue) 1 else 0)