syscall: fix epoll_event struct for ppc64le/ppc64

The existing epoll_event structure used by many of
the epoll_* syscalls was defined incorrectly
for use with ppc64le & ppc64 in the syscall
directory.  This resulted in the caller getting
incorrect information on return from these
syscalls.  This caused failures in fsnotify as
well as builds with upstream Docker.  The
structure is defined correctly in gccgo.

This adds a pad field that is expected for
these syscalls on ppc64le, ppc64.
Fixes #15135

Fixes #15288

Change-Id: If7e8ea9eb1d1ca5182c8dc0f935b334127341ffd
Reviewed-on: https://go-review.googlesource.com/21582


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/22207


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>