unix: fix infinite recursion in itoa

Bring in http://golang.org/cl/138650044 from syscall package.

Change-Id: I554b0b31b981c682b6826644564971321a55c9e4
Reviewed-on: https://go-review.googlesource.com/10030
Reviewed-by: Rob Pike <r@golang.org>

unix/syscall_test.go

@@ -7,6 +7,7 @@
 package unix_test
 
 import (
+	"fmt"
 	"testing"
 
 	"golang.org/x/sys/unix"
@@ -31,3 +32,19 @@ func TestEnv(t *testing.T) {
 	// make sure TESTENV gets set to "", not deleted
 	testSetGetenv(t, "TESTENV", "")
 }
+
+func TestItoa(t *testing.T) {
+	// Make most negative integer: 0x8000...
+	i := 1
+	for i<<1 != 0 {
+		i <<= 1
+	}
+	if i >= 0 {
+		t.Fatal("bad math")
+	}
+	s := unix.Itoa(i)
+	f := fmt.Sprint(i)
+	if s != f {
+		t.Fatalf("itoa(%d) = %s, want %s", i, s, f)
+	}
+}