AArch64: Refactor err_type.

AArch64: Refactor err_type.

Previously the ERR_ values were defined as different constants, to make this a
bit more type safe and so they can be more easily re-used I'm changing them into
an actual enum and updating any usages.

include/

	* opcode/aarch64.h (enum err_type): New.
	(aarch64_decode_insn): Use it.

opcodes/

	* aarch64-dis.c (ERR_OK, ERR_UND, ERR_UNP, ERR_NYI): Remove.
	(aarch64_decode_insn, print_insn_aarch64_word): Use err_type.