eventfd.h: Use new "bits/" scheme for arch-specific flags

eventfd.h: Use new "bits/" scheme for arch-specific flags

As in timerfd.h, eventfd.h needs arch-specific definition files.
alpha, mips and sparc needs separate file, all the other arch
will use common definition.

This problem is already fixed in glibc.

Also sanitize and provide bits for hppa.
Make sure not to install the new bits/eventfd unless eventfd support is
enabled.

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>