linux-user: correctly align target_epoll_event

According to comments in /usr/include/linux/eventpoll.h,
poll_event is packed only on x86_64.

And to be sure fields are correctly aligned in epoll_data,
use abi_XXX types for all of them.

Moreover, fd type is wrong: fd is int, not ulong.

This has been tested with a ppc guest on an x86_64 host:
without this patch, systemd crashes (core).

CC: Alexander Graf <agraf@suse.de>
CC: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
