syscall: add a padding field to EpollEvent on s390x

Fixes #16021.

Change-Id: I55df38bbccd2641abcb54704115002a9aa04325d
Reviewed-on: https://go-review.googlesource.com/23962
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/types_linux.go

@@ -117,7 +117,7 @@ struct my_epoll_event {
 	// alignment requirements of EABI
 	int32_t padFd;
 #endif
-#ifdef  __powerpc64__
+#if defined(__powerpc64__) || defined(__s390x__)
 	int32_t _padFd;
 #endif
 	int32_t fd;

src/syscall/ztypes_linux_s390x.go

@@ -588,6 +588,7 @@ type Ustat_t struct {
 
 type EpollEvent struct {
 	Events uint32
+	_      int32
 	Fd     int32
 	Pad    int32
 }