atomic: Consolidate atomic_full_barrier implementation

atomic: Consolidate atomic_full_barrier implementation

All ABIs save for sparcv9 and s390 defines it to __sync_synchronize,
which can be mapped to __atomic_thread_fence (__ATOMIC_SEQ_CST).

For Sparc, it uses a stricter #StoreStore|#LoadStore|#StoreLoad|#LoadLoad
instead of the #StoreLoad generated by __sync_synchronize.

For s390x, it defaults to a memory barrier where __sync_synchronize
emits a 'bcr 15,0' (which the manual describes as pipeline synchronization).

The barrier is used only in one place (pthread_mutex_setprioceiling),
and using a stricter barrier for s390 is ok performance-wise.

Co-authored-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>