posix: pthread: reimplement pthread_barrier using zephyr objects

Previously pthread_barrier_t was implemented in terms of wait
queues and internal scheduler functions.

This introduced some obstacles and inconsistency. In order
to be more consistent, rely only on Zephyr's public API and
reuse as many concepts as possible.

Deprecate `PTHREAD_BARRIER_DEFINE()` since it's non-standard.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>