MFC r260401

  Remove aicasm as a build dependency.  It made sense when the ahc and ahd
  drivers and their firmware were under active development, but those days
  have passed.  The firmware now exists in pre-compiled form, no longer
  dependent on it's sources or on aicasm.  If you wish to rebuild the
  firmware from source, the glue still exists under the 'make firmware'
  target in sys/modules/aic7xxx.

  This also fixes the problem introduced with r257777 et al with building
  kernels the old fashioned way in sys/$arch/compile/$CONFIG when the
  ahc/ahd drivers were included.