Generalize inline support for __cpp__ magic to other targets

[andyli]

Inline support for __cpp__ magic introduced in #2875 should be supported by other targets.

e.g. for __js__:

class Global {
    static public function set_a(v:Int):Void {
        untyped __js__("a = {0}", v);
    }
}

[Simn]

@waneck: Could you move the code you introduced in #2875 to codegen.ml and call it from the generators? From a quick look that should be straightforward.

[Simn]

I've done that and allowed it for __js__. Some tests would be appreciated.

[Simn]

I'll just assume this works.