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

[Andrew Cagney]

> 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.

Just FYI, the process for obsoleting a target is defined here:

http://sources.redhat.com/gdb/current/onlinedocs/gdbint_15.html#SEC134

(it should mention to update NEWS ...)

Andrew