clock: Test timerfd's support for monotonic clock

In tst-timerfd, test timerfd with monotonic clock, in addition to the
existing test with the realtime clock.

This patch also changes this test to only use Linux APIs, not anything
OSv-specific, so it can also be run on Linux.

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>