init: Add last_{init, exit} list macros

Add a last_init macro for constructor functions that will be executed last
in the initialization process. Add a symmetrical macro, last_exit, for
destructor functions that will be the last to be executed when kvmtool
exits.

The list priority for the late_{init, exit} macros has been bumped down a
spot, but their relative priority remains unchanged, to keep the same size
for the init_lists and exit_lists.