[maint] Drop z8k, h8500 and mn10200 from build list?

[Michael Elizabeth Chastain]

My nefarious idea:

          z8k             --OBSOLETE.t.a.r.g.e.t=z8k-coff ,-Werror

So the information would be human-accessible during the obsoletion
phase in case anyone steps forward to rescue it.  But gdb_mbuild.sh
would not pick it up.

Michael C