sizeof(struct epoll_event) wrong on amd64

The size of 'struct epoll_event' is wrong on AMD64 (16 bytes vs 12 bytes),
which causes some programs to crash (for example nginx).

The definition of 'struct epoll_event' in the spec (and in the actual LSB
headers) is missing a packed attribute:
http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-AMD64/LSB-Core-AMD64/libc-ddefs.html
http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/libc-ddefs.html

In the kernel 'struct epoll_event' is expected to be packed on AMD64:
http://lxr.linux.no/#linux+v3.11/include/uapi/linux/eventpoll.h

[ I filed the bug against Core-Arch, but maybe its easier to just fix this in
the generic header with an ifdef like in the kernel header ]

I've checked the latest headers according to instructions from another
bugreport, and the bug is still there:
bzr branch http://bzr.linuxfoundation.org/lsb/devel/build_env/

$ cat >epolltest.c <<EOF
#include <sys/epoll.h>
#include <stdio.h>
int main()
{
    printf("sizeof(struct epoll_event)=%d\n", sizeof(struct epoll_event));
    return 0;
}
EOF

$ /opt/lsb/bin/lsbcc epolltest.c && ./a.out
sizeof(struct epoll_event)=16

$ gcc epolltest.c && ./a.out
sizeof(struct epoll_event)=12
[reply] [-] Comment 1