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_linux.go

@@ -13,6 +13,7 @@ package unix
 
 import (
 	"encoding/binary"
+	"strconv"
 	"syscall"
 	"time"
 	"unsafe"
@@ -233,7 +234,7 @@ func Futimesat(dirfd int, path string, tv []Timeval) error {
 func Futimes(fd int, tv []Timeval) (err error) {
 	// Believe it or not, this is the best we can do on Linux
 	// (and is what glibc does).
-	return Utimes("/proc/self/fd/"+itoa(fd), tv)
+	return Utimes("/proc/self/fd/"+strconv.Itoa(fd), tv)
 }
 
 const ImplementsGetwd = true