cmd/compile/internal/base: replace io/ioutil with os package

Change-Id: Ide7618b65fcff7947809051d51b993d330e35717
GitHub-Last-Rev: 298853a820
GitHub-Pull-Request: golang/go#55914
Reviewed-on: https://go-review.googlesource.com/c/go/+/435537
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/base/flag.go

@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"internal/buildcfg"
 	"internal/coverage"
-	"io/ioutil"
 	"log"
 	"os"
 	"reflect"
@@ -437,7 +436,7 @@ func readImportCfg(file string) {
 
 func readCoverageCfg(file string) {
 	var cfg coverage.CoverFixupConfig
-	data, err := ioutil.ReadFile(file)
+	data, err := os.ReadFile(file)
 	if err != nil {
 		log.Fatalf("-coveragecfg: %v", err)
 	}