[kernel] Stop creating user stacks in the kernel

Stop creating stacks in user space for user threads, that should be done
by the thread's creator. This change adds process and stack_top
arguments to the thread_create syscall, so that threads can be created
in other processes, and given a stack address.

Also included is a fix in add_thunk_user due to the r11/flags change.

THIS COMMIT BREAKS USERSPACE. See subsequent commits for the user side
changes related to this change.

src/kernel/objects/process.h

@@ -43,10 +43,10 @@ public:
     vm_space & space() { return m_space; }
 
     /// Create a new thread in this process
+    /// \args rsp3      If non-zero, sets the ring3 stack pointer to this value
     /// \args priority  The new thread's scheduling priority
-    /// \args user      If true, create a userspace stack for this thread
     /// \returns        The newly created thread object
-    thread * create_thread(uint8_t priorty = default_priority, bool user = true);
+    thread * create_thread(uintptr_t rsp3 = 0, uint8_t priorty = default_priority);
 
     /// Start tracking an object with a handle.
     /// \args obj  The object this handle refers to