cgo: use C type void for opaque types if dwarf.Common().Type returns nothing

Ideally, the C name would come from the typedef or pointer that
references the Size<0 type, but we can't easily generate this without
performing a look-ahead to see if any referencing type will become opaque.

Fixes #334.
Fixes #281.

R=rsc
https://golang.org/cl/161056

src/cmd/cgo/gcc.go

@@ -320,6 +320,9 @@ func (c *typeConv) Type(dtype dwarf.Type) *Type {
 		// Unsized types are [0]byte
 		t.Size = 0;
 		t.Go = c.Opaque(0);
+		if t.C == "" {
+			t.C = "void"
+		}
 		return t;
 	}