CPU: Let each MIPSState own its JIT

The global MIPSComp::jit becomes MIPSState::jit, created and deleted by the
state it runs. JitAt() compiles with the JIT of the state it's handed, so
the dispatchers no longer go through a global. Code that is about the main
CPU by nature (the UI, the crash handler, the disassembler's symbol lookup,
the JIT harness) uses mipsr4k.jit.

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

Core/HLE/sceKernelThread.cpp

@@ -1453,9 +1453,9 @@ void __KernelLoadContext(const PSPThreadContext *ctx, bool vfpuEnabled) {
 
 	memcpy(currentMIPS->other, ctx->other, sizeof(ctx->other));
 	// Not locking here, we assume the jit isn't switched during execution.
-	if (MIPSComp::jit) {
+	if (currentMIPS->jit) {
 		// When thread switching, we must update the rounding mode if cached in the jit.
-		MIPSComp::jit->UpdateFCR31();
+		currentMIPS->jit->UpdateFCR31();
 	}
 
 	// Reset the llBit, the other thread may have touched memory.