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.
300bf9c2c5