x/sys/unix: add Dup2 wrapper for arm64/ppc64

arm64 doesn't have a Dup2 syscall, instead Dup3 is supposed
to be used. Since Dup3 is linux-specific, provide a wrapper
to make writing portable code easier.

    Updates golang/go#10235

To verify it, added a testcase for Dup and Dup2.

Change-Id: I066bb60d62b2bd64d7ba0fdfbb334ce2213c78e9
Reviewed-on: https://go-review.googlesource.com/20178
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_unix_test.go

@@ -316,3 +316,38 @@ func TestSeekFailure(t *testing.T) {
 		t.Fatalf("Seek(-1, 0, 0) return error with empty message")
 	}
 }
+
+func TestDup(t *testing.T) {
+	file, err := ioutil.TempFile("", "TestDup")
+	if err != nil {
+		t.Fatalf("Tempfile failed: %v", err)
+	}
+	defer os.Remove(file.Name())
+	defer file.Close()
+	f := int(file.Fd())
+
+	newFd, err := unix.Dup(f)
+	if err != nil {
+		t.Fatalf("Dup: %v", err)
+	}
+
+	err = unix.Dup2(newFd, newFd+1)
+	if err != nil {
+		t.Fatalf("Dup2: %v", err)
+	}
+
+	b1 := []byte("Test123")
+	b2 := make([]byte, 7)
+	_, err = unix.Write(newFd+1, b1)
+	if err != nil {
+		t.Fatalf("Write to dup2 fd failed: %v", err)
+	}
+	_, err = unix.Seek(f, 0, 0)
+	_, err = unix.Read(f, b2)
+	if err != nil {
+		t.Fatalf("Read back failed: %v", err)
+	}
+	if string(b1) != string(b2) {
+		t.Errorf("Dup: stdout write not in file, expected %v, got %v", string(b1), string(b2))
+	}
+}