cmd/link: disallow linkname of runtime.addmoduledata

Although a comment on addmoduledata warns that it should not be
called from Go code, the linker still allow it to be accessed via
go:linkname. Using linkname on this function will cause a linker
crash when building with buildmode=plugin.

Fixes #75180

Change-Id: Ib72c367a8afaef712ca5e29b1d0a4fc98ed8f40f
Reviewed-on: https://go-review.googlesource.com/c/go/+/699655
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/link/internal/loader/loader.go

@@ -2440,6 +2440,7 @@ var blockedLinknames = map[string][]string{
 	// Others
 	"net.newWindowsFile":                   {"net"},              // pushed from os
 	"testing/synctest.testingSynctestTest": {"testing/synctest"}, // pushed from testing
+	"runtime.addmoduledata":                {},                   // disallow all package
 }
 
 // check if a linkname reference to symbol s from pkg is allowed

src/cmd/link/link_test.go

@@ -1613,6 +1613,7 @@ func TestCheckLinkname(t *testing.T) {
 		{"coro2.go", false},
 		// pull linkname of a builtin symbol is not ok
 		{"builtin.go", false},
+		{"addmoduledata.go", false},
 		// legacy bad linkname is ok, for now
 		{"fastrand.go", true},
 		{"badlinkname.go", true},

src/cmd/link/testdata/linkname/addmoduledata.go

@@ -0,0 +1,18 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Linkname runtime.addmoduledata is not allowed.
+
+package main
+
+import (
+	_ "unsafe"
+)
+
+//go:linkname addmoduledata runtime.addmoduledata
+func addmoduledata()
+
+func main() {
+	addmoduledata()
+}