Workaround TSan false positive in AO_stack_pop_explicit_aux_acquire

Workaround TSan false positive in AO_stack_pop_explicit_aux_acquire
(fix commits c058d9d, 6ffda1db)

The TSan-related workaround in AO_malloc is no longer needed.

* src/atomic_ops_malloc.c [AO_THREAD_SANITIZER
&& AO_USE_ALMOST_LOCK_FREE] (AO_malloc): Do not use AO_store (always
use *result=log_sz); remove comment.
* src/atomic_ops_stack.c [AO_USE_ALMOST_LOCK_FREE
&& AO_THREAD_SANITIZER] (AO_load_next): New static function (with
AO_ATTR_NO_SANITIZE_THREAD attribute); add comments.
* src/atomic_ops_stack.c [AO_USE_ALMOST_LOCK_FREE
&& !AO_THREAD_SANITIZER] (AO_load_next): Define to AO_load.
* src/atomic_ops_stack.c [AO_USE_ALMOST_LOCK_FREE]
(AO_stack_pop_explicit_aux_acquire): Replace AO_load(first_ptr) with
AO_load_next(first_ptr).