Kernel: Use Userspace<T> for the exit_thread syscall

Userspace<void*> is a bit strange here, as it would appear to the
user that we intend to de-refrence the pointer in kernel mode.

However I think it does a good join of illustrating that we are
treating the void* as a value type,  instead of a pointer type.

Kernel/Syscall.cpp

@@ -95,7 +95,7 @@ int handle(RegisterState& regs, u32 function, u32 arg1, u32 arg2, u32 arg3)
         if (function == SC_exit)
             process.sys$exit((int)arg1);
         else
-            process.sys$exit_thread((void*)arg1);
+            process.sys$exit_thread(arg1);
         ASSERT_NOT_REACHED();
         return 0;
     }