[atomicsabi64] Add _Atomic types (#353)

Define size/alignment of _Atomic types. Define atomic_is_lock_free
result.

---------

Co-authored-by: Wilco Dijkstra <[email protected]>

Author: Wilco1

atomicsabi64/atomicsabi64.rst

@@ -379,6 +379,15 @@ Suggestions and improvements to this specification may be submitted to the:
 `issue tracker page on GitHub <https://github.com/ARM-software/abi-aa/issues>`_.
 
 
+Atomic types
+============
+
+``_Atomic`` struct types types with size less than 16 bytes must be padded to the nearest power
+of 2.  Their alignment must be the same as their size so that they can be used by atomic instructions.
+
+``atomic_is_lock_free`` must return ``true`` for all ``_Atomic`` objects with size less than or equal
+to 16 bytes, and ``false`` otherwise.
+
 
 AArch64 atomic mappings
 =======================