Set bfd_gas for all sparc targets. Added facility for keeping or rejecting

Set bfd_gas for all sparc targets.  Added facility for keeping or rejecting
configurations still under development; default is to assume production
environment, and reject configs still being worked on.  Mark Elf configurations
(except sparc and i386) as developmental.  Deleted cases matching some generic
names in favor of more specific names.  (E.g., when we get Alpha
support, we'll still only support VMS for Vax.)