cmd/cgo: ignore vars with no name or type if they have a AttrSpecification

Fixes #13344.

Change-Id: I33c6721fd33d144c85c87840ddf27ce15aa72328
Reviewed-on: https://go-review.googlesource.com/17151
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

misc/cgo/test/issue1635.go

@@ -14,6 +14,11 @@ void scatter() {
 	printf("scatter = %p\n", p);
 }
 
+// Adding this explicit extern declaration makes this a test for
+// https://gcc.gnu.org/PR68072 aka https://golang.org/issue/13344 .
+// It used to cause a cgo error when building with GCC 6.
+extern int hola;
+
 // this example is in issue 3253
 int hola = 0;
 int testHola() { return hola; }

src/cmd/cgo/gcc.go

@@ -491,6 +491,11 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 			name, _ := e.Val(dwarf.AttrName).(string)
 			typOff, _ := e.Val(dwarf.AttrType).(dwarf.Offset)
 			if name == "" || typOff == 0 {
+				if e.Val(dwarf.AttrSpecification) != nil {
+					// Since we are reading all the DWARF,
+					// assume we will see the variable elsewhere.
+					break
+				}
 				fatalf("malformed DWARF TagVariable entry")
 			}
 			if !strings.HasPrefix(name, "__cgo__") {