Add PPC_MAX_REGISTER_SIZE

Add PPC_MAX_REGISTER_SIZE

gdb/
	* ppc-linux-nat.c (fetch_register): Use PPC_MAX_REGISTER_SIZE.
	(store_register): Likewise.
	* ppc-sysv-tdep.c (ppc_sysv_abi_push_dummy_call): Likewise.
	(get_decimal_float_return_value): Likewise.
	(do_ppc_sysv_return_value): Likewise.
	(ppc64_sysv_abi_push_integer): Likewise.
	(ppc64_sysv_abi_push_freg): Likewise.
	(ppc64_sysv_abi_return_value_base): Likewise.
	(ppc64_sysv_abi_return_value): Likewise.
	* rs6000-aix-tdep.c (rs6000_push_dummy_call): Likewise.
	* rs6000-lynx178-tdep.c (rs6000_lynx178_push_dummy_call): Likewise.
	* rs6000-nat.c: Likewise.
	* rs6000-tdep.c (rs6000_register_to_value): Likewise.
	(rs6000_value_to_register): Likewise.
	* ppc-tdep.h (PPC_MAX_REGISTER_SIZE): Add.