us our first minimal glimpse of PowerPC support.
With this code we can get to the "mountroot>" prompt on my Apple iMac. We
can't get any further due to lack of clock and interrupt handling, among other
things. This does however mean that pmap and VM are initialising.
We're fairly dependant on OpenFirmware at this point, but I hope to add
support for other classes of firmware at a later stage.
Reviewed by: obrien, dfr
