Define new tag for atomic ABI.

Alternative version of #383

Defined 4 atomic abi variant.

- unknown, nonatomic, A6C and A6S

Unknown is compatilbe with exising object file.
Nonatomic is used when A-extension is absent
A6C is the table A.6 mapping defined in ISA spec.
And the A6S is the mapping defined in psABI spec.