unix: fix EpollWait for arm64

epoll_wait syscall doesn't exist on arm64. Implement it with
by callign epoll_pwait(). According to man epoll_pwait,
calling epoll_pwait with sigmask of NULL is identical to
epoll_wait.

Testing exposed that EpollEvent needs padding on arm64
like on arm.

This changeset is to fix:
https://github.com/fsnotify/fsnotify/issues/130

Testcase: go test with fsnotify ported from syscall to x/sys:
https://github.com/suihkulokki/fsnotify/tree/go-sys

Change-Id: I76136bf4c82c2ee597549133848f490da46dd488
Reviewed-on: https://go-review.googlesource.com/21971
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/zsyscall_linux_mips64.go b/unix/zsyscall_linux_mips64.go
index 7247005..9b52d60 100644
--- a/unix/zsyscall_linux_mips64.go
+++ b/unix/zsyscall_linux_mips64.go
@@ -370,23 +370,6 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error) {
-	var _p0 unsafe.Pointer
-	if len(events) > 0 {
-		_p0 = unsafe.Pointer(&events[0])
-	} else {
-		_p0 = unsafe.Pointer(&_zero)
-	}
-	r0, _, e1 := Syscall6(SYS_EPOLL_WAIT, uintptr(epfd), uintptr(_p0), uintptr(len(events)), uintptr(msec), 0, 0)
-	n = int(r0)
-	if e1 != 0 {
-		err = errnoErr(e1)
-	}
-	return
-}
-
-// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
-
 func Exit(code int) {
 	Syscall(SYS_EXIT_GROUP, uintptr(code), 0, 0)
 	return
@@ -1216,6 +1199,23 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error) {
+	var _p0 unsafe.Pointer
+	if len(events) > 0 {
+		_p0 = unsafe.Pointer(&events[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero)
+	}
+	r0, _, e1 := Syscall6(SYS_EPOLL_WAIT, uintptr(epfd), uintptr(_p0), uintptr(len(events)), uintptr(msec), 0, 0)
+	n = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fchown(fd int, uid int, gid int) (err error) {
 	_, _, e1 := Syscall(SYS_FCHOWN, uintptr(fd), uintptr(uid), uintptr(gid))
 	if e1 != 0 {