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/hle/kernel/kernel.cpp

@@ -153,12 +153,8 @@ void Shutdown() {
  * @return True on success, otherwise false
  */
 bool LoadExec(u32 entry_point) {
-    Core::g_app_core->SetPC(entry_point);
-
     // 0x30 is the typical main thread priority I've seen used so far
-    g_main_thread = Kernel::SetupMainThread(0x30, Kernel::DEFAULT_STACK_SIZE);
-    // Setup the idle thread
-    Kernel::SetupIdleThread();
+    g_main_thread = Kernel::SetupMainThread(Kernel::DEFAULT_STACK_SIZE, entry_point, 0x30);
 
     return true;
 }