DolphinQt: Properly lock CPU before accessing emulated memory

This fixes a problem I was having where using frame advance with the
debugger open would frequently cause panic alerts about invalid addresses
due to the CPU thread changing MSR.DR while the host thread was trying
to access memory.

To aid in tracking down all the places where we weren't properly locking
the CPU, I've created a new type (in Core.h) that you have to pass as a
reference or pointer to functions that require running as the CPU thread.

Source/Core/Core/IOS/IOS.cpp

@@ -17,6 +17,7 @@
 #include "Common/CommonTypes.h"
 #include "Common/Logging/Log.h"
 #include "Common/Timer.h"
+
 #include "Core/Boot/AncastTypes.h"
 #include "Core/Boot/DolReader.h"
 #include "Core/Boot/ElfReader.h"
@@ -912,7 +913,10 @@ static void FinishPPCBootstrap(Core::System& system, u64 userdata, s64 cycles_la
   else
     ReleasePPC();
 
-  SConfig::OnNewTitleLoad();
+  ASSERT(Core::IsCPUThread());
+  Core::CPUThreadGuard guard;
+  SConfig::OnNewTitleLoad(guard);
+
   INFO_LOG_FMT(IOS, "Bootstrapping done.");
 }