Add OpenBSD/powerpc support.

Add OpenBSD/powerpc support.
* NEWS (New native configurations): Mention OpenBSD/powerpc.
* ppc-tdep.h (struct ppc_reg_offsets): New structure.
(ppc_supply_gregset, ppc_supply_fpregset, ppc_collect_gregset)
(ppc_collect_fpregset): New prototypes.
* ppcobsd-nat.c: New file.
* ppcobsd-tdep.c: New file.
* ppcobsd-tdep.h: New file.
* rs6000-tdep.c: Include "regset.h".
(ppc_supply_reg, ppc_collect_reg, ppc_supply_gregset)
(ppc_supply_fpregset, ppc_collect_gregset, ppc_collect_fpregset):
New functions.
* Makefile.in (ALLDEPFILES): Add pccobsd-nat.c and ppcobsd-tdep.c.
(ppcobsd-nat.o, ppcobsd-tdep.o): New dependencies.
(rs6000-tdep.o): Update dependencies.
* configure.host: Add powerpc-*-openbsd.
* configure.tgt: Add powerpc-*-openbsd.
* config/powerpc/obsd.mh: New file.
* config/powerpc/nm-obsd.h: New file.
* config/powerpc/obsd.mt: New file.