CPU: Convert state enum to an enum class

Gets enum constants out of the immediate namespace. Also makes it
strongly typed like the other state enums.

Source/Core/Core/PowerPC/PowerPC.h

@@ -152,7 +152,7 @@ const char* GetCPUName();
 // Init() will be called when added and Shutdown() when removed.
 // [Threadsafety: Same as SetMode(), except it cannot be called from inside the CPU
 //  run loop on the CPU Thread - it doesn't make sense for a CPU to remove itself
-//  while it is CPU_RUNNING]
+//  while it is in State::Running]
 void InjectExternalCPUCore(CPUCoreBase* core);
 
 // Stepping requires the CPU Execution lock (CPU::PauseAndLock or CPU Thread)