core: implement ppoll()

A simple implementation for ppoll() using poll().
Minoru says ppoll() is needed in programs such as TCPServer.

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
libc: add eaccess() to run irb

eaccess() refer to access(2)

Signed-off-by: KANATSU Minoru <icc.pot.tyew272@gmail.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>