Make FreezableAtomicReference atomic when unfrozen for the new MM.

kotlin-native/backend.native/tests/runtime/workers/freeze2.kt

@@ -59,36 +59,27 @@ data class Data(var int: Int)
     assertFailsWith<InvalidMutabilityException> { a8[1] = 2.0 }
 
     // Ensure that String and integral boxes are frozen by default, by passing local to the worker.
-    val hasToBeFrozen = Platform.memoryModel == MemoryModel.STRICT
     val worker = Worker.start()
     var data: Any = "Hello" + " " + "world"
-    if (hasToBeFrozen) {
-        assertTrue(data.isFrozen)
-    }
+    assertTrue(data.isFrozen)
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker 1: $input")
     }.result
 
     data = 42
-    if (hasToBeFrozen) {
-        assertTrue(data.isFrozen)
-    }
+    assertTrue(data.isFrozen)
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker2: $input")
     }.result
 
     data = 239.0
-    if (hasToBeFrozen) {
-        assertTrue(data.isFrozen)
-    }
+    assertTrue(data.isFrozen)
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker3: $input")
     }.result
 
     data = 'a'
-    if (hasToBeFrozen) {
-        assertTrue(data.isFrozen)
-    }
+    assertTrue(data.isFrozen)
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker4: $input")
     }.result