unix: use strconv.Itoa instead of local implementation

This was originally copied over from package syscall where it was
replaced by internal/itoa in CL 301549.

For golang.org/x/sys/unix we may import strconv, so use strconv.Itoa
instead.

Change-Id: Iac125fbd0f64c385f9f0c02d4a7af762364b67aa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/425304
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

unix/syscall_test.go

@@ -8,7 +8,6 @@
 package unix_test
 
 import (
-	"fmt"
 	"testing"
 
 	"golang.org/x/sys/unix"
@@ -34,22 +33,6 @@ func TestEnv(t *testing.T) {
 	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)
-	}
-}
-
 func TestUname(t *testing.T) {
 	var utsname unix.Utsname
 	err := unix.Uname(&utsname)