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/str.go

@@ -8,8 +8,12 @@ package unix
 
 func itoa(val int) string { // do it here rather than with fmt to avoid dependency
 	if val < 0 {
-		return "-" + itoa(-val)
+		return "-" + uitoa(uint(-val))
 	}
+	return uitoa(uint(val))
+}
+
+func uitoa(val uint) string {
 	var buf [32]byte // big enough for int64
 	i := len(buf) - 1
 	for val >= 10 {