utils/eventfd: honor EFD_NONBLOCK as a bitflag on macOS

`EventFd::new(EFD_NONBLOCK | EFD_SEMAPHORE)` left the pipe blocking, since
the flag was compared for equality. `pollable_channel` passes exactly that
combination, so its `try_recv()` would block on an empty pipe while holding
the queue mutex.

Signed-off-by: vadik likholetov <vadikas@gmail.com>