[HXCPP] When targeting abstract implementation class, Reflect.hasField returns true but Reflect.field returns null

My current objective is to create a system which enables my implementation of HScript to access constants and functions from abstracts.

My current implementation works as follows:

- Using `haxe.macro.Context.onGenerate`, get the list of types at macro time.
- For each type which is a `TAbstract`, record the path of the abstract and the implementation class into a `Map>` which I make available at runtime. For example, `flixel.util.FlxColor` (an abstract which wraps the Int type) gets mapped to `flixel.util._FlxColor.FlxColor_Impl_`.
- I then can use `Reflect` to access the static fields of the class.

The issue is that this has odd behavior depending on the platform.

On JavaScript targets, I can use `Reflect.fields()` to get the list of static fields, then use `Reflect.field(name)` to retrieve the value of a field by name. This works on both constants (`Reflect.field(FlxColor, 'BLUE')` returns an integer constant corresponding to `FlxColor.BLUE`) and functions (`Reflect.field(FlxColor, 'fromHexString')` returns a Function which I can use with `Reflect.callMethod()`).

On HXCPP, I can use `Reflect.fields()` and this gives the full list of static fields (both static variables and static methods), but the internal function `__GetStatic` which powers the reflection call only includes the static methods, so `Reflect.field(FlxColor, 'fromHexString')` works but `Reflect.field(FlxColor, 'BLUE')` does not.

When investigating this, I found this section of code to be the core issue:

https://github.com/HaxeFoundation/haxe/blob/39aceb1e45f9bdc9f5a7e07aa891f72a51a0029b/src/generators/gencpp.ml#L6268

I believe the issue is that the `is_readable` function, which filters the values listed by `__GetStatic()`, hides all the static constants of the abstract.

I'm looking to see if modifying this condition causes any problems. If it doesn't, I can make a pull request, but if it does (or you have some issue with that change) we can discuss a way that I can still use Reflection to access these fields.

EDIT: This was tested on Haxe 4.3.6