Make SingleThreadMarkAndSweep support multiple threads

kotlin-native/backend.native/tests/objcexport/values.swift

@@ -1073,6 +1073,11 @@ class TestSharedRefs {
             }
         }
 
+        // This will free `object1` and release+dealloc its associated `Deinit` which nils `Deinit.object2`
+        ValuesKt.gc()
+        // This will free `object2`.
+        ValuesKt.gc()
+
         try assertTrue(Deinit.weakVar2 === nil)
     }
 
@@ -1403,10 +1408,7 @@ class ValuesTests : SimpleTestProvider {
         test("TestInvalidIdentifiers", testInvalidIdentifiers)
         test("TestDeprecation", testDeprecation)
         test("TestWeakRefs", testWeakRefs)
-        if !ValuesKt.isExperimentalMM {
-            // Experimental MM doesn't support multiple mutators yet.
-            test("TestSharedRefs", TestSharedRefs().test)
-        }
+        test("TestSharedRefs", TestSharedRefs().test)
         test("TestClassTypeCheck", testClassTypeCheck)
         test("TestInterfaceTypeCheck", testInterfaceTypeCheck)
         test("TestGH3503_1", testGH3503_1)
@@ -1419,9 +1421,6 @@ class ValuesTests : SimpleTestProvider {
         test("TestFakeOverrideInInterface", testFakeOverrideInInterface)
 
         // Stress test, must remain the last one:
-        if !ValuesKt.isExperimentalMM {
-            // Experimental MM doesn't support multiple mutators yet.
-            test("TestGH2931", testGH2931)
-        }
+        test("TestGH2931", testGH2931)
     }
 }