Add OpenBSD/m68k support.

Add OpenBSD/m68k support.
* NEWS (New native configurations): Mention OpenBSD/m68k.
* m68kbsd-tdep.c: New file.
* m68kbsd-nat.c: New file.
* Makefile.in (ALLDEPFILES): Add m68kbsd-nat.c and m68kbsd-tdep.c.
(m68kbsd-nat.o, m68kbsd-tdep.o): New dependencies.
* configure.host: Add m68k-*-openbsd.
* configure.tgt: Add m68k-*-openbsd.
* config/m68k/tm-obsd.h: New file.
* config/m68k/obsd.mt: New file.
* config/m68k/obsd.mh: New file.
* config/djgpp/fnchange.lst: Add entries for m68kbsd-nat.c and
m68kbsd-tdep.c.