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/CachedInterpreter/CachedInterpreter.cpp

@@ -107,7 +107,7 @@ void CachedInterpreter::ExecuteOneBlock()
 
 void CachedInterpreter::Run()
 {
-  while (CPU::GetState() == CPU::CPU_RUNNING)
+  while (CPU::GetState() == CPU::State::Running)
   {
     // Start new timing slice
     // NOTE: Exceptions may change PC