libc: implement ffsl and ffsll functions

We already have the ffs function, ffsl and ffsll are easy from here.
Theoretically, a_ctz_l should do the job here as well since we're 64 all
over, but I found a_ctz_64 safer.

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