[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.

definitions/objects/thread.def

@@ -2,6 +2,8 @@ object thread : kobject {
     uid 11f23e593d5761bd
 
     method create [constructor] {
+        param process object process
+        param stack_top address
         param entrypoint address
     }