sched: reserve some thread ids

This patch reserves some thread ids, that are kept unused. This is so we can
construct values that reuse the thread public id and add it together with other
information and still fit in 32-bits.

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