Kernel: Use Userspace<T> for the join_thread syscall

Kernel/Process.h

@@ -306,7 +306,7 @@ public:
     int sys$sched_getparam(pid_t pid, Userspace<struct sched_param*>);
     int sys$create_thread(void* (*)(void*), Userspace<const Syscall::SC_create_thread_params*>);
     void sys$exit_thread(void*);
-    int sys$join_thread(pid_t tid, void** exit_value);
+    int sys$join_thread(pid_t tid, Userspace<void**> exit_value);
     int sys$detach_thread(pid_t tid);
     int sys$set_thread_name(pid_t tid, Userspace<const char*> buffer, size_t buffer_size);
     int sys$get_thread_name(pid_t tid, Userspace<char*> buffer, size_t buffer_size);

Kernel/Syscalls/thread.cpp

@@ -117,7 +117,7 @@ int Process::sys$detach_thread(pid_t tid)
     return 0;
 }
 
-int Process::sys$join_thread(pid_t tid, void** exit_value)
+int Process::sys$join_thread(pid_t tid, Userspace<void**> exit_value)
 {
     REQUIRE_PROMISE(thread);
     if (exit_value && !validate_write_typed(exit_value))