[cpp] Add cpp::AtomicInt type

std/cpp/AtomicInt.hx

@@ -0,0 +1,18 @@
+package cpp;
+
+@:scalar @:coreType
+extern abstract AtomicInt from(Int) to(Int)
+{
+   // returns true if exchange took place
+   @:native("_hx_atomic_exchange_if")
+   public static function exchangeIf(ioValue:Pointer<AtomicInt>, test:Int, newVal:Int) : Bool;
+
+   // returns value before increment
+   @:native("_hx_atomic_inc")
+   public static function atomicInc(ioValue:Pointer<AtomicInt>) : Int;
+
+   // returns value before decrement
+   @:native("_hx_atomic_dec")
+   public static function atomicDec(ioValue:Pointer<AtomicInt>) : Int;
+}
+