Move dumplo from MD to kern/kern_shutdown.c

Move dumplo from MD to kern/kern_shutdown.c

Move db_disasm.c from the platform architecture to the cpu architecture

Move add missing __volatile to cpu_halt()'s HLT function to prevent it from
being optimized out.

Remove the vm_page_zero_idle() function (that has not been used for a while).

Move some more function prototypes for MD functions used by MI code into MI
header files.