pthread: facilitate dynamically allocated thread stacks

This change allows users to call pthread_create() with
the pthread_attr_t argument equal to NULL.

If Zephyr is configured with `CONFIG_DYNAMIC_THREAD`, then a
suitable thread stack will be allocated via
k_thread_stack_alloc(). The allocated thread stack is
automatically freed via k_thread_stack_free().

This makes the Zephyr implementation of pthread_create()
compliant with the normative spec.

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>