Add a new ram0 pseudo-device that claims memory resouces for physical

Add a new ram0 pseudo-device that claims memory resouces for physical
addresses corresponding to system RAM.  On amd64 ram0 uses the SMAP
and claims all the type 1 SMAP regions.  On i386 ram0 uses the
dump_avail[] array.  Note that on i386 we have to ignore regions above
4G in PAE kernels since bus resources use longs.