[js] only generate Math.imul usage for haxe.Int32 if we actually used haxe.Int32.mul

std/haxe/Int32.hx

@@ -200,7 +200,7 @@ abstract Int32(Int) from Int to Int {
 
 	#if js
 	static function __init__() {
-		untyped if (Math.imul != null) Int32.mul = Math.imul;
+		untyped __feature__("haxe._Int32.Int32_Impl_.mul", if (Math.imul != null) Int32.mul = Math.imul);
 	}
 	#end
 }