libaio: stub more functions

Stub more libaio functions to warn and abort.

Usually, these additional stub functions aren't necessary, because when
an application calls io_setup() and fails, it won't call one of these
other functions. However, for an application compiled with on-load symbol
resolution, it can be important to have these additional symbols as well.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Stub signalfd()

This is a silly warn-and-fail implementation for the Linux signalfd()
system call. It's obviously not useful for applications that really need
signalfd(), but it is useful for applications which can gracefully fail
without it (or not call it at all), but still need this symbol to be
available (e.g., because on-load symbol resolution was enabled on the
object).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>