- add a RAS hook in cpu_switch().
- fill in the definition of struct mcontext.
- implement cpu_upcall(), cpu_getmcontext(), cpu_getmcontext() and
cpu_switchto().
- for now, force the right priviledge bits and space regs in setcontext().
- use correct values for __SIMPLELOCK_*.
- move the user stack to start at a multiple of the pthread stack size
so that libpthread can use the sp-masking trick.
