opcodes: aarch64: flags to denote subclasses of ldst insns

[Changes in V4]
 - Specify subclasses only for those iclasses relevent to SCFI:
      ldst_imm9, ldst_pos, ldstpair_indexed, ldstpair_off
[End of changes in V4]

[Changes in V3]
- Use F_LDST_SWAP for lse_atomic ld/st ops.  Use of F_LDST_LOAD or
  F_LDST_STORE was incorrect.
[End of changes in V3]

[New in V2]

The existing iclass information tells us the general shape and purpose
of the instructions.  In some cases, however, we need to further disect
the iclass on the basis of other finer-grain information.  E.g., for the
purpose of SCFI, we need to know whether a given insn with iclass
of ldst_* is a load or a store.

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