additional processors are spun up on multiprocessor Alpha systems.
Now, each processor gets its own idle thread (the primary processor
uses proc0). This idle thread is used in switch_exit(), rather than
explicitly referencing proc0.
Also, make `curproc', `fpcurproc', and `curpcb' per-cpu values. This
required some data structure rearrangement; cpu info is now statically
allocated in the BSS, rather than via malloc(), and cpu_softc is gone.
(Modeled somewhat after NetBSD/sparc's multiprocessor info structures.)
thorpej