pthread: Add pthread_attr_getstacksize()

Add pthread_attr_getstacksize() in preparation for fixing RLIMIT_STACK.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>