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.

core/os/thread_dummy.cpp

@@ -40,14 +40,6 @@ void ThreadDummy::make_default() {
 	Thread::create_func = &ThreadDummy::create;
 };
 
-SemaphoreOld *SemaphoreDummy::create() {
-	return memnew(SemaphoreDummy);
-};
-
-void SemaphoreDummy::make_default() {
-	SemaphoreOld::create_func = &SemaphoreDummy::create;
-};
-
 RWLock *RWLockDummy::create() {
 	return memnew(RWLockDummy);
 };