internal/coverage/cfile: remove //go:linkname into testing

This is just looking at a command-line flag. Look directly.

For #67401.

Change-Id: I7a1c3fc2d9cc85e5ffc5731444bf6db87abf6901
Reviewed-on: https://go-review.googlesource.com/c/go/+/585916
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/internal/coverage/cfile/ts_test.go

@@ -6,6 +6,7 @@ package cfile
 
 import (
 	"encoding/json"
+	"flag"
 	"internal/coverage"
 	"internal/goexperiment"
 	"internal/testenv"
@@ -17,13 +18,11 @@ import (
 	_ "unsafe"
 )
 
-//go:linkname testing_testGoCoverDir testing.testGoCoverDir
-func testing_testGoCoverDir() string
-
 func testGoCoverDir(t *testing.T) string {
-	tgcd := testing_testGoCoverDir()
-	if tgcd != "" {
-		return tgcd
+	if f := flag.Lookup("test.gocoverdir"); f != nil {
+		if dir := f.Value.String(); dir != "" {
+			return dir
+		}
 	}
 	return t.TempDir()
 }

src/testing/newcover.go

@@ -46,16 +46,6 @@ func coverReport2() {
 	}
 }
 
-// testGoCoverDir is used in runtime/coverage tests.
-//go:linkname testGoCoverDir
-
-// testGoCoverDir returns the value passed to the -test.gocoverdir
-// flag by the Go command, if goexperiment.CoverageRedesign is
-// in effect.
-func testGoCoverDir() string {
-	return *gocoverdir
-}
-
 // coverage2 returns a rough "coverage percentage so far"
 // number to support the testing.Coverage() function.
 func coverage2() float64 {