Scheduler refactor Pt. 1

* Simplifies scheduling logic, specifically regarding thread status. It should be much clearer which statuses are valid
for a thread at any given point in the system.
* Removes dead code from thread.cpp.
* Moves the implementation of resetting a ThreadContext to the corresponding core's implementation.

Other changes:
* Fixed comments in arm interfaces.
* Updated comments in thread.cpp
* Removed confusing, useless, functions like MakeReady() and ChangeStatus() from thread.cpp.
* Removed stack_size from Thread. In the CTR kernel, the thread's stack would be allocated before thread creation.

src/core/arm/arm_interface.h

@@ -85,6 +85,15 @@ public:
      */
     virtual void AddTicks(u64 ticks) = 0;
 
+    /**
+     * Initializes a CPU context for use on this CPU
+     * @param context Thread context to reset
+     * @param stack_top Pointer to the top of the stack
+     * @param entry_point Entry point for execution
+     * @param arg User argument for thread
+     */
+    virtual void ResetContext(Core::ThreadContext& context, u32 stack_top, u32 entry_point, u32 arg) = 0;
+
     /**
      * Saves the current CPU context
      * @param ctx Thread context to save