unix: fix EpollEvent padding on mips64{,le}

Like on other 64-bit GOARCHes, EpollEvent needs padding before Fd for
mips64 and mips64le.

Change-Id: I934368ba22ef7bf3a25949649613c2a03928bf83
Reviewed-on: https://go-review.googlesource.com/c/sys/+/207278
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/types.go

@@ -283,7 +283,8 @@ struct my_epoll_event {
 	// padding is not specified in linux/eventpoll.h but added to conform to the
 	// alignment requirements of EABI
 	int32_t padFd;
-#elif defined(__powerpc64__) || defined(__s390x__) || defined(__sparc__) || defined(__riscv)
+#elif defined(__powerpc64__) || defined(__s390x__) || defined(__sparc__) || defined(__riscv) \
+		|| (defined(__mips__) && _MIPS_SIM == _MIPS_SIM_ABI64)
 	int32_t _padFd;
 #endif
 	int32_t fd;

unix/ztypes_linux_mips64.go

@@ -783,6 +783,7 @@ type Ustat_t struct {
 
 type EpollEvent struct {
 	Events uint32
+	_      int32
 	Fd     int32
 	Pad    int32
 }

unix/ztypes_linux_mips64le.go

@@ -783,6 +783,7 @@ type Ustat_t struct {
 
 type EpollEvent struct {
 	Events uint32
+	_      int32
 	Fd     int32
 	Pad    int32
 }