CPU: Pass the MIPSState explicitly where it was already at hand

The JIT backends' calls out of generated code (the MIPS and IR interpreter
fallbacks, and the bad address report) now take the state the backend was
built for instead of reading currentMIPS. Likewise the interpreter's tick
check, the IR dispatcher's Advance, and a couple of debugger helpers.

MIPSState::InitRegisters() no longer sets currentMIPS; the boot paths set it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/System.cpp

@@ -548,6 +548,7 @@ static bool CPU_Init(FileLoader *fileLoader, IdentifiedFileType type, std::strin
 	LoadSymbolsIfSupported();
 	LoadGameSymbolsIfEnabled();
 
+	currentMIPS = &mipsr4k;
 	mipsr4k.Reset();
 
 	CoreTiming::Init(&mipsr4k);