libc: use poll_wake() in eventfd implementation

Nadav pointed out that 79fa4e8adf9b3065ea7f9c039f396ffa78f3e8ff ("libc:
add poll_[un]install() functions to event_fd") basically duplicates much
of already implemented poll() logic. What eventfd was actually lacking
before that patch were calls to poll_wake() in appropriate places.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>