Allow "ddb{1}> trace" through interrupt on macppc

Allow "ddb{1}> trace" through interrupt on macppc

If cpu0 sends PPC_IPI_DDB to cpu1, then cpu1 stops on its interrupt
stack.  Teach ININTSTK to allow traces through all interrupt stacks,
not only cpu0's.

ININTSTK now works by looping for all cpus.  It doesn't remember which
cpu owns the stack.  A macppc has at most 4 cpus.

ok kettenis@ miod@