drivers: net: nsos: implement poll() syscall

Use NSI_HW_EVENT() in order to periodically check for events in host
sockets. Whenever there is a socket event ready to be processed by Zephyr,
raise native_sim (newly introduced) CPU interrupt, so that Zephyr driver
can signal readiness with k_poll().

Maintain a list of Zephyr poll() executions in Zephyr context. Iterate
through them whenever there is some event to be processed.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>