Adapt to new auxiliary stack argument of `libuksched` API

Adapt to new auxiliary stack argument of `libuksched` API

Now all thread creation related functions also require an auxiliary
stack-related argument: either a pointer to it or its desired length.

Thus, provide argument 0 for length of auxiliary stack, allowing the
allocation to use the default length.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #41