Kernel+UserspaceEmulator: Add sys$emuctl() system call

This returns ENOSYS if you are running in the real kernel, and some
other result if you are running in UserspaceEmulator.

There are other ways we could check if we're inside an emulator, but
it seemed easier to just ask. :^)

Kernel/Process.h

@@ -211,6 +211,7 @@ public:
     void stop_tracing();
     void tracer_trap(Thread&, const RegisterState&);
 
+    KResultOr<int> sys$emuctl();
     KResultOr<int> sys$yield();
     KResultOr<int> sys$sync();
     KResultOr<int> sys$beep();