gh-159603: Use pthread_attr_get_np() on FreeBSD (#159604)

Use pthread_attr_get_np() on FreeBSD to get the stack size (and guard
size) of the current thread.

Check if pthread_attr_get_np() function and <pthread_np.h> header are
available in configure.

Update test_json.test_recursion. On FreeBSD 16 with a default stack size of
512 MB, the json C implementation no longer raises RecursionError in
test_highly_nested_objects_encoding(). Reduce also max_depth from 5,000 to
2,000 so test_highly_nested_objects_encoding() of the Python implementation
only takes 1.6 seconds, instead of 12 seconds.