sched: expose thread_remote_local_var to outside users

While working with blocked signals and notifications, it would be good to test
what's the current state of other thread's pending signal mask.

That machinery exists in sched.cc but isn't exposed. This patch exposes that,
together with a more convenient helper for when we are interested in the
pointer itself, without dereferencing it.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>