Transition to a model where the loader passes the address of the
bootinfo block in register r8. In locore.s we save the address
in the global variable 'pa_bootinfo'. In machdep.c we compare
this value against the hardwired address, but don't depend on its
validity yet (ie: we still expect the bootinfo block to be at the
hardwired address). After a small amount of time, we'll flip the
switch and depend on the loader to pass us the address. From that
moment on the loader is free to put it anywhere it likes, provided
the machine itself likes it as well.

Add some verbosity to aid in the transition. We emit a message if
the loader didn't pass the address and we also emit a message if
there's no bootinfo block at the hardwired address.

While in locore.s, reduce the number of redundant serialization
instructions. A srlz.i is a proper superset of a srlz.d and thus
is a valid replacement. Also slightly reorder the movl instructions
to improve bundle density.