posix: pthread: add option for pthread_create() barrier

To enable testing, introduce `CONFIG_PTHREAD_CREATE_BARRIER`.

Some observations were made that running several Qemu SMP targets
concurrently could lead to synchronization problems. On such
targets, it was found that the synchronization issues were
mitigated by introducing a `pthread_barrier_t` shared between
`pthread_create()` and the spawned thread.

It is suggested to enable the option when running many
SMP tests concurrently in several parallel Qemu processes,
e.g. with `twister`.

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