Kernel/Synch: Do not attempt a reschedule on every syscall.

Not all syscalls should cause reschedules, this commit attempts to remedy that, however, it still does not cover all cases.

src/core/hle/kernel/mutex.cpp

@@ -84,6 +84,7 @@ void Mutex::Release() {
         if (lock_count == 0) {
             holding_thread->held_mutexes.erase(this);
             ResumeWaitingThread(this);
+            Core::System::GetInstance().PrepareReschedule();
         }
     }
 }