Use auto_obstack in objfile

This changes objfile to use an auto_obstack.  This helps prevent
use-after-free bugs, because it ensures that anything allocated on the
objfile obstack will live past the point at which the registry object
is destroyed.

gdb/objfiles.h

@@ -666,7 +666,7 @@ public:
   /* Obstack to hold objects that should be freed when we load a new symbol
      table from this object file.  */
 
-  struct obstack objfile_obstack {};
+  auto_obstack objfile_obstack;
 
   /* Structure which keeps track of functions that manipulate objfile's
      of the same type as this objfile.  I.e. the function to read partial