cmd/link: handle dynamic import variables on Darwin in plugin mode

CL 501855 added support for cgo_dynamic_import variables on Darwin.
But it didn't support the plugin build mode on amd64, where the
assembler turns a direct load (R_PCREL) to a load via GOT
(R_GOTPCREL). This CL adds the support. We just need to handle
external linking mode, as this can only occur in plugin or shared
build mode, which requires external linking.

Fixes #67976.
Updates #50891.

Change-Id: I0f56265d50bfcb36047fa5538ad7a5ec77e7ef96
Reviewed-on: https://go-review.googlesource.com/c/go/+/592499
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/internal/testplugin/plugin_test.go

@@ -414,3 +414,11 @@ func TestTextSectionSplit(t *testing.T) {
 		t.Errorf("runtime.text.1 not found, text section not split?")
 	}
 }
+
+func TestIssue67976(t *testing.T) {
+	// Issue 67976: build failure with loading a dynimport variable (the runtime/pprof
+	// package does this on darwin) in a plugin on darwin/amd64.
+	// The test program uses runtime/pprof in a plugin.
+	globalSkip(t)
+	goCmd(t, "build", "-buildmode=plugin", "-o", "issue67976.so", "./issue67976/plugin.go")
+}

src/cmd/cgo/internal/testplugin/testdata/issue67976/plugin.go

@@ -0,0 +1,16 @@
+// Copyright 2024 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.
+
+package main
+
+import (
+	"io"
+	"runtime/pprof"
+)
+
+func main() {}
+
+func Start() {
+	pprof.StartCPUProfile(io.Discard)
+}

src/cmd/link/internal/amd64/asm.go

@@ -398,6 +398,13 @@ func adddynrel(target *ld.Target, ldr *loader.Loader, syms *ld.ArchSyms, s loade
 			// (e.g. go version).
 			return true
 		}
+	case objabi.R_GOTPCREL:
+		if target.IsExternal() {
+			// External linker will do this relocation.
+			return true
+		}
+		// We only need to handle external linking mode, as R_GOTPCREL can
+		// only occur in plugin or shared build modes.
 	}
 
 	return false