[js] use Math.imul for haxe.Int32 multiplication

std/haxe/Int32.hx

@@ -197,4 +197,8 @@ abstract Int32(Int) from Int to Int {
 		return (x);
 		#end
 	}
+
+	static function __init__() {
+		untyped if (Math.imul != null) Int32.mul = Math.imul;
+	}
 }