Make SingleThreadMarkAndSweep support multiple threads

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

@@ -59,27 +59,36 @@ 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"
-    assert(data.isFrozen)
+    if (hasToBeFrozen) {
+        assertTrue(data.isFrozen)
+    }
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker 1: $input")
     }.result
 
     data = 42
-    assert(data.isFrozen)
+    if (hasToBeFrozen) {
+        assertTrue(data.isFrozen)
+    }
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker2: $input")
     }.result
 
     data = 239.0
-    assert(data.isFrozen)
+    if (hasToBeFrozen) {
+        assertTrue(data.isFrozen)
+    }
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker3: $input")
     }.result
 
     data = 'a'
-    assert(data.isFrozen)
+    if (hasToBeFrozen) {
+        assertTrue(data.isFrozen)
+    }
     worker.execute(TransferMode.SAFE, { data } ) {
         input -> println("Worker4: $input")
     }.result
@@ -87,4 +96,4 @@ data class Data(var int: Int)
     worker.requestTermination().result
 
     println("OK")
-}
+}