libc: missing __isnanf

Implement that, together with the ones we already have.

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>
pthread: implement missing scope functions

Those functions set and get the scheduling scope of a thread. Luckily for us,
out of the two scheduling scopes defined, Linux only supports one
(PTHREAD_SCOPE_SYSTEM). The implementation of these functions is therefore
trivial.

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>