cmd/link: handle -w flag in external linking mode

Currently, when the -w flag is set, it doesn't actually disable
the debug info generation with in external linking mode. (It does
make the Go object have no debug info, but C objects may still
have.) Pass "-Wl,-S" to let the external linker disable debug info
generation.

Change-Id: I0fce56b9f23a45546b69b9e6dd027c5527b1bc87
Reviewed-on: https://go-review.googlesource.com/c/go/+/705857
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/dwarf_test.go

@@ -370,14 +370,26 @@ func TestFlagW(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	tests := []struct {
+	type testCase struct {
 		flag      string
 		wantDWARF bool
-	}{
+	}
+	tests := []testCase{
 		{"-w", false},     // -w flag disables DWARF
 		{"-s", false},     // -s implies -w
 		{"-s -w=0", true}, // -w=0 negates the implied -w
 	}
+	if testenv.HasCGO() {
+		tests = append(tests,
+			testCase{"-w -linkmode=external", false},
+			testCase{"-s -linkmode=external", false},
+			// Some external linkers don't have a way to preserve DWARF
+			// without emitting the symbol table. Skip this case for now.
+			// I suppose we can post- process, e.g. with objcopy.
+			//testCase{"-s -w=0 -linkmode=external", true},
+		)
+	}
+
 	for _, test := range tests {
 		name := strings.ReplaceAll(test.flag, " ", "_")
 		t.Run(name, func(t *testing.T) {

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

@@ -1451,6 +1451,8 @@ func (ctxt *Link) hostlink() {
 		} else {
 			argv = append(argv, "-s")
 		}
+	} else if *FlagW {
+		argv = append(argv, "-Wl,-S") // suppress debugging symbols
 	}
 
 	// On darwin, whether to combine DWARF into executable.