runtime: force the type of specialfinalizer into DWARF

The core dump reader wants to know the layout of this type.
No variable has this type, so it wasn't previously dumped
to DWARF output.

Change-Id: I982040b81bff202976743edc7fe53247533a9d81
Reviewed-on: https://go-review.googlesource.com/68312Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mheap.go

@@ -160,6 +160,8 @@ type mheap struct {
 	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
 	specialprofilealloc   fixalloc // allocator for specialprofile*
 	speciallock           mutex    // lock for special record allocators.
+
+	unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
 }
 
 var mheap_ mheap