The only usage was for pthread_spin_lock, introduced by 12d2dd706099aa4,
as a way to optimize the code for certain architectures. Now that atomic
builtins are used by default, let the compiler use the best code sequence
for the atomic exchange.

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Now that atomic builtins are used by default, we can rely on the
compiler to define when to use 64-bit atomic operations.

It allows the use of 64-bit atomic operations on some 32-bit ABIs where
they were not previously enabled due to missing pre-processor handling:
hppa, mips64n32, s390, and sparcv9.

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

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>

There is no Linux specific definitions.