spt: Fix hang in solo5_yield

[reynir]

Same fix as in 97a66f6

Calling solo5_yield(0, ...) results in passing a struct itimerspec with an it_value of zero in both fields to timerfd_settime(), which disarms the timer, which causes the subsequent call to epoll_pwait() to hang forever if no other file descriptors (external events) have been registered with the waitset.

This addresses mirage/mirage-solo5#97

Dune cache was interfering and this patch only worked once I disabled the dune cache.

[Firobe]

I confirm this fixes the issue, thank you :)

[reynir]

I think it could be nicer both in spt and hvt to check if deadline is zero and then call epoll_pwait() with 0 as timeout instead of -1, but it requires a bit more code and I don't feel confident with that task.