xfs: use generic get_unaligned_beXX helpers

Switch the shortform directory code over to use the generic
get_unaligned_beXX helpers instead of reinventing them.  As a result
kill off xfs_arch.h and move the setting of XFS_NATIVE_HOST into
xfs_linux.h.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>