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.
b7f4763aaf