Drop old semaphore implementation

- Removed platform-specific implementations.
- Now all semaphores are in-object, unless they need to be conditionally created.
- Similarly to `Mutex`, provided a dummy implementation for when `NO_THREADS` is defined.
- Similarly to `Mutex`, methods are made `const` for easy use in such contexts.
- Language bindings updated: `wait()` and `post()` are now `void`.
- Language bindings updated: `try_wait()` added.

Bonus:
- Rewritten the `#ifdef` in `mutex.h` to meet the code style.

platform/osx/SCsub

@@ -9,7 +9,6 @@ files = [
     'crash_handler_osx.mm',
     'os_osx.mm',
     'godot_main_osx.mm',
-    'semaphore_osx.cpp',
     'dir_access_osx.mm',
     'joypad_osx.cpp',
     'vulkan_context_osx.mm',