Add arm64 support. On ARM write-combining translates into the normal uncached

Add arm64 support.  On ARM write-combining translates into the normal uncached
memory attribute and uncached translates into device-nGnRnE memory.  This
complicates the mapping onto PMAP_WC, PMAP_NOCACHE and PMAP_DEVICE a bit
since the requirements of the drm(4) subsystem don't quite match the natural
definitions for these.

ok jsg@, mpi@, visa@