a pointer to current cpu's cpu_info structure. Use cpu_info for
intstk,intr_depth,still_stk,idle_pcb,curpcb,curlwp,etal even on
non-MULTIPROCESSOR machines. Add common macros GET_CPUINFO and
INIT_CPUINFO to get and initialize the cpu_info struct on startup. Make
ibm4xx use the standard <powerpc/frame.h>. Use IFRAME_xx in ibm4xx
trap_subr.S instead of explicit magic offsets. Move INTSTK and SPILLSTK
to std.<platform>. Change faultbuf to a struct instead of an array.
On MPC6XX cpus, stop using the vector page for temporary space and use
reserved space in cpu_info.
