[kernel] Give threads initial arguments

This commit changes the add_user_thunk to point to a new routine,
initialize_user_cpu, which sets all the registers that were previously
unset when starting a new user thread. The values for rdi and rsi are
popped off the initial stack values that add_user_thunk sets up, so that
user thread procs can take up to two arguments.

To suppor this, j6_thread_create gained two new arguments, which are
passed on to the thread.

This also let me finally get rid of the hack of passing an argument in
rsp when starting init.

src/libraries/j6/thread.cpp

@@ -31,7 +31,7 @@ thread::thread(thread::proc p, uintptr_t stack_top) :
 }
 
 j6_status_t
-thread::start()
+thread::start(void *user)
 {
     if (m_status != j6_status_ok)
         return m_status;
@@ -39,8 +39,12 @@ thread::start()
     if (m_thread != j6_handle_invalid)
         return j6_err_invalid_arg;
 
+    uint64_t arg0 = reinterpret_cast<uint64_t>(this);
+    uint64_t arg1 = reinterpret_cast<uint64_t>(user);
+
     m_status = j6_thread_create(&m_thread, __handle_self,
-        m_stack_top, reinterpret_cast<uintptr_t>(m_proc));
+        m_stack_top, reinterpret_cast<uintptr_t>(init_proc),
+        arg0, arg1);
 
     return m_status;
 }
@@ -51,6 +55,13 @@ thread::join()
     j6_thread_join(m_thread);
 }
 
+void
+thread::init_proc(thread *t, void *user)
+{
+    t->m_proc(user);
+    j6_thread_exit();
 }
 
+} // namespace j6
+
 #endif // __j6kernel