CPU: Implement instruction cache simulation

Implemented for all execution modes. Disabled by default in the cached
interpreter and recompiler, always enabled in the pure interpreter.

src/core/settings.h

@@ -69,6 +69,7 @@ struct Settings
 
   CPUExecutionMode cpu_execution_mode = CPUExecutionMode::Interpreter;
   bool cpu_recompiler_memory_exceptions = false;
+  bool cpu_recompiler_icache = false;
 
   float emulation_speed = 1.0f;
   bool speed_limiter_enabled = true;