mips64: Add a Dup2 compatibility function, as it is not implemented in that
arch.

--- a/unix/syscall_linux_mips64x.go
+++ b/unix/syscall_linux_mips64x.go
@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build linux
-// +build mips64 mips64le
+// +build mips64 mips64le mipsn64
 
 package unix
 
@@ -206,3 +206,7 @@
 	}
 	return poll(&fds[0], len(fds), timeout)
 }
+
+func Dup2(oldfd int, newfd int) (err error) {
+	return Dup3(oldfd, newfd, 0)
+}