TODO file: move item about sync-base primitives to aarch64.h and generic.h
Use __atomic GCC built-in to implement generic double-wide CAS

* src/atomic_ops/sysdeps/gcc/generic.h (AO_double_compare_and_swap):
Implement using __atomic_compare_exchange_n(__ATOMIC_RELAXED) instead
of __sync_bool_compare_and_swap.