Use __HAVE_NEW_PMAP_68K for all virt68k.

If the loader passed us a RAM disk, use it as the root file system.

Add a port to the Qemu m68k "virt" platform, a 68040-based virtual machine
with VirtIO I/O devices.

This is a work-in-progress, but "successfully mounts root file system
and then complains about missing /sbin/init that is, in fact, not there"
seems like a good checkpoint.