unix: remove the use of ioutil

Trivial changes.

Change-Id: If7ffa47bb4c3b2bae784091e94516b0f4fc197d2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526300
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>

unix/linux/mkall.go

@@ -24,7 +24,6 @@ import (
 	"fmt"
 	"go/build/constraint"
 	"io"
-	"io/ioutil"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -810,7 +809,7 @@ func generatePtraceRegSet(arch string) error {
 // ptraceDef returns the definition of PtraceRegs for arch.
 func ptraceDef(arch string) (string, error) {
 	filename := fmt.Sprintf("ztypes_linux_%s.go", arch)
-	data, err := ioutil.ReadFile(filename)
+	data, err := os.ReadFile(filename)
 	if err != nil {
 		return "", fmt.Errorf("reading %s: %v", filename, err)
 	}