cmd/go: remove -w workaround for -buildmode=plugin on Darwin

The problem causing the assert in #21647 are fixed at this point,
along with various other linker issues with plugin + Darwin. With
this in mind, remove the "-ldflags=-w" workaround for plugin mode on
Darwin and re-enable the appropriate tests misc/cgo/testplugin

Fixes #21647.
Fixes #27502.

Change-Id: I5b662987b138b06cfc9e1f9f6d804cf682bd501a
Reviewed-on: https://go-review.googlesource.com/c/go/+/206198Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

misc/cgo/testplugin/plugin_test.go

@@ -14,7 +14,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -114,11 +113,7 @@ func run(t *testing.T, bin string, args ...string) string {
 
 func TestDWARFSections(t *testing.T) {
 	// test that DWARF sections are emitted for plugins and programs importing "plugin"
-	if runtime.GOOS != "darwin" {
-		// On macOS, for some reason, the linker doesn't add debug sections to .so,
-		// see issue #27502.
-		goCmd(t, "run", "./checkdwarf/main.go", "plugin2.so", "plugin2.UnexportedNameReuse")
-	}
+	goCmd(t, "run", "./checkdwarf/main.go", "plugin2.so", "plugin2.UnexportedNameReuse")
 	goCmd(t, "run", "./checkdwarf/main.go", "./host.exe", "main.main")
 }
 

src/cmd/go/internal/work/init.go

@@ -200,8 +200,6 @@ func buildModeInit() {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x", "linux/ppc64le",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
 			case "darwin/amd64":
-				// Skip DWARF generation due to #21647
-				forcedLdflags = append(forcedLdflags, "-w")
 			case "freebsd/amd64":
 			default:
 				base.Fatalf("-buildmode=plugin not supported on %s\n", platform)