Revert "cmd/link: prefix syms with "_" on external darwin links"

This reverts CL 194381

Reason for revert: break tests like add2line.

Change-Id: I9e858c7ada340a842bd0cad719616ad30fae4aaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/195137Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/link/internal/ld/issue33808_test.go

-// Copyright 2019 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 ld
-
-import (
-	"internal/testenv"
-	"io/ioutil"
-	"os"
-	"runtime"
-	"strings"
-	"testing"
-)
-
-const prog = `
-package main
-
-import "log"
-
-func main() {
-	log.Fatalf("HERE")
-}
-`
-
-func TestIssue33808(t *testing.T) {
-	if runtime.GOOS != "darwin" {
-		return
-	}
-	testenv.MustHaveGoBuild(t)
-
-	dir, err := ioutil.TempDir("", "TestIssue33808")
-	if err != nil {
-		t.Fatalf("could not create directory: %v", err)
-	}
-	defer os.RemoveAll(dir)
-
-	f := gobuild(t, dir, prog, "-ldflags=-linkmode=external")
-	f.Close()
-
-	syms, err := f.Symbols()
-	if err != nil {
-		t.Fatalf("Error reading symbols: %v", err)
-	}
-
-	name := "log.Fatalf"
-	for _, sym := range syms {
-		if strings.Contains(sym.Name, name) {
-			return
-		}
-	}
-	t.Fatalf("Didn't find %v", name)
-}

src/cmd/link/internal/ld/macho.go

@@ -869,7 +869,6 @@ func machosymtab(ctxt *Link) {
 		symtab.AddUint32(ctxt.Arch, uint32(symstr.Size))
 
 		export := machoShouldExport(ctxt, s)
-		isGoSymbol := strings.Contains(s.Extname(), ".")
 
 		// In normal buildmodes, only add _ to C symbols, as
 		// Go symbols have dot in the name.
@@ -878,8 +877,8 @@ func machosymtab(ctxt *Link) {
 		// symbols like crosscall2 are in pclntab and end up
 		// pointing at the host binary, breaking unwinding.
 		// See Issue #18190.
-		cexport := !isGoSymbol && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
-		if cexport || export || isGoSymbol {
+		cexport := !strings.Contains(s.Extname(), ".") && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
+		if cexport || export {
 			symstr.AddUint8('_')
 		}