cmd/link: write dwarf sections

Also stop skipping TestExternalLinkerDWARF and
TestDefaultLinkerDWARF.

Fixes #10776.

Change-Id: Ia596a684132e3cdee59ce5539293eedc1752fe5a
Reviewed-on: https://go-review.googlesource.com/36983Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1482,7 +1482,11 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 	}
 
 	if Linkmode == LinkExternal {
-		if !Iself && Headtype != obj.Hdarwin {
+		switch {
+		case Iself:
+		case Headtype == obj.Hdarwin:
+		case Headtype == obj.Hwindows:
+		default:
 			return
 		}
 	}

src/debug/pe/file_cgo_test.go

@@ -19,7 +19,6 @@ func testCgoDWARF(t *testing.T, linktype int) {
 }
 
 func TestDefaultLinkerDWARF(t *testing.T) {
-	t.Skip("skipping broken test: see issue 10776")
 	testCgoDWARF(t, linkCgoDefault)
 }
 
@@ -28,6 +27,5 @@ func TestInternalLinkerDWARF(t *testing.T) {
 }
 
 func TestExternalLinkerDWARF(t *testing.T) {
-	t.Skip("skipping broken test: see issue 10776")
 	testCgoDWARF(t, linkCgoExternal)
 }