unix: add missing dup2 syscall on mips64*

After this commit the testsuite passes on mips64le.

Fixes golang/go#20340.

Change-Id: I6b3f673674af107fd6d249ccf5e695e0e50a41bb
Reviewed-on: https://go-review.googlesource.com/43430
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: jcowgill

unix/syscall_linux_mips64x.go

@@ -7,6 +7,7 @@
 
 package unix
 
+//sys	Dup2(oldfd int, newfd int) (err error)
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)