* aix-thread.c (supply_gprs64, fetch_regs_user_thread)

(store_regs_user_thread): Use tdep->ppc_gp0_regnum, instead
of assuming that the gprs are numbered starting with zero.
* ppc-linux-nat.c (fill_gregset): Same.
* ppc-linux-tdep.c (ppc_linux_supply_gregset): Same.
* ppcnbsd-nat.c (getregs_supplies): Same.
* ppcnbsd-tdep.c (ppcnbsd_supply_reg, ppcnbsd_fill_reg): Same.
* rs6000-nat.c (fetch_core_registers): Same.
* rs6000-tdep.c (ppc_supply_gregset, ppc_collect_gregset): Same.