cmd/nm: accept macho files which don't have symbol table in the archive

After https://golang.org/cl/64793, we started to include Mach-O object
files which don't have symbol table into cgo archive.
However, toolchains didn't handle those files yet.

Fixes #21959

Change-Id: Ibb2f6492f1fa59368f2dfd4cff19783997539875
Reviewed-on: https://go-review.googlesource.com/65170Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/objfile/macho.go

@@ -30,7 +30,7 @@ func openMacho(r io.ReaderAt) (rawFile, error) {
 
 func (f *machoFile) symbols() ([]Sym, error) {
 	if f.macho.Symtab == nil {
-		return nil, fmt.Errorf("missing symbol table")
+		return nil, nil
 	}
 
 	// Build sorted list of addresses of all symbols.

src/cmd/nm/nm.go

@@ -110,15 +110,19 @@ func nm(file string) {
 
 	entries := f.Entries()
 
+	var found bool
+
 	for _, e := range entries {
 		syms, err := e.Symbols()
 		if err != nil {
 			errorf("reading %s: %v", file, err)
 		}
 		if len(syms) == 0 {
-			errorf("reading %s: no symbols", file)
+			continue
 		}
 
+		found = true
+
 		switch *sortOrder {
 		case "address":
 			sort.Slice(syms, func(i, j int) bool { return syms[i].Addr < syms[j].Addr })
@@ -155,5 +159,9 @@ func nm(file string) {
 		}
 	}
 
+	if !found {
+		errorf("reading %s: no symbols", file)
+	}
+
 	w.Flush()
 }