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/PowerPC/Interpreter/Interpreter.cpp

@@ -14,6 +14,7 @@
 #include "Common/Logging/Log.h"
 #include "Common/StringUtil.h"
 #include "Core/Config/MainSettings.h"
+#include "Core/Core.h"
 #include "Core/CoreTiming.h"
 #include "Core/Debugger/Debugger_SymbolMap.h"
 #include "Core/HLE/HLE.h"
@@ -339,11 +340,14 @@ void Interpreter::Run()
 
 void Interpreter::unknown_instruction(UGeckoInstruction inst)
 {
-  const u32 opcode = PowerPC::HostRead_U32(last_pc);
+  ASSERT(Core::IsCPUThread());
+  Core::CPUThreadGuard guard;
+
+  const u32 opcode = PowerPC::HostRead_U32(guard, last_pc);
   const std::string disasm = Common::GekkoDisassembler::Disassemble(opcode, last_pc);
   NOTICE_LOG_FMT(POWERPC, "Last PC = {:08x} : {}", last_pc, disasm);
-  Dolphin_Debugger::PrintCallstack(Core::System::GetInstance(), Common::Log::LogType::POWERPC,
-                                   Common::Log::LogLevel::LNOTICE);
+  Dolphin_Debugger::PrintCallstack(Core::System::GetInstance(), guard,
+                                   Common::Log::LogType::POWERPC, Common::Log::LogLevel::LNOTICE);
   NOTICE_LOG_FMT(
       POWERPC,
       "\nIntCPU: Unknown instruction {:08x} at PC = {:08x}  last_PC = {:08x}  LR = {:08x}\n",

Source/Core/Core/PowerPC/Interpreter/Interpreter_Branch.cpp

@@ -6,6 +6,7 @@
 #include "Common/Assert.h"
 #include "Common/CommonTypes.h"
 #include "Core/ConfigManager.h"
+#include "Core/Core.h"
 #include "Core/HLE/HLE.h"
 #include "Core/PowerPC/Interpreter/ExceptionUtils.h"
 #include "Core/PowerPC/PowerPC.h"
@@ -95,7 +96,11 @@ void Interpreter::bclrx(UGeckoInstruction inst)
 void Interpreter::HLEFunction(UGeckoInstruction inst)
 {
   m_end_block = true;
-  HLE::Execute(PowerPC::ppcState.pc, inst.hex);
+
+  ASSERT(Core::IsCPUThread());
+  Core::CPUThreadGuard guard;
+
+  HLE::Execute(guard, PowerPC::ppcState.pc, inst.hex);
 }
 
 void Interpreter::rfi(UGeckoInstruction inst)