clock: Use new clock APIs in alarm() implementation

Change alarm() implementation to use the new <osv/clock.hh> APIs and the
monotonic clock.

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