CMAKE: Emscripten shared lib hack is now resolved upstream

While working on the CMake solution I had to implement a hack to enable the production of shared libs.

As part of the initial investigation effort I had subscribed to the bug reports and PR's and so today I received an email notifying that the emscripten tickets are closed, and that emscripten toolchain now interprets the `-shared` flag to mean building side modules.

[PR I had subscribed to](https://github.com/emscripten-core/emscripten/pull/16281)
[resolution](https://github.com/emscripten-core/emscripten/pull/25930)

So I think this means that at some point in the future the following things will happen (when they happen):
- emscripten tags a release with this change
- godot starts using that release
- godot-cpp can then drop the use of the emscripten hack.

This issue is to flag these changes, so that when the above happens, there is at least some guidance.

If anyone is more experienced in such matters, please correct.