enabled on amd64). Add a dmat64 field to various PCI attach structures,
and pass it down where needed. Implement a simple new function called
pci_dma64_available(pa) to test if 64bit DMA addresses may be used.
This returns 1 iff _PCI_HAVE_DMA64 is defined in <machine/pci_machdep.h>,
and there is more than 4G of memory.
Make ALLOCNOW the default iff bouncing might be needed (this has
no effect on i386 because ISA DMA devices already had to use
ALLOCNOW, and PCI isn't bounced (yet), since we don't do > 4G
at this point for i386.
