opcodes: aarch64: add flags to denote subclasses of arithmetic insns

opcodes: aarch64: add flags to denote subclasses of arithmetic insns

Use the three new subclass flags: F_ARITH_ADD, F_ARITH_SUB,
F_ARITH_MOV, to indicate add, sub and mov ops respectively.

These flags for subclasses will later be used for SCFI purposes to
create appropriate ginsns.  At this time, only those iclasses relevant
to SCFI have the new subclass flags specified.

For addg and subg insns, F_SUBCLASS_OTHER is more suitable because these
operations do more than just simple add or sub.

opcodes/
    * aarch64-tbl.h: Use the new F_ARITH_* flags.