gdb: fix owner passed to remove_target_sections in clear_solib

Commit 8971d2788e ("gdb: link so_list using intrusive_list") introduced
a bug in clear_solib.  Instead of passing an `so_list *` to
remove_target_sections, it passed an `so_list **`.  This was not caught
by the compiler, because remove_target_sections takes a `void *` as the
"owner", so you can pass it any pointer and it won't complain.

This happened because I previously had a patch to change the type of the
disposer parameter to be a reference rather than a pointer, so had to
change `so` to `&so`.  When dropping that patch, I forgot to revert this
bit and / or it got re-introduced when handling subsequent merge
conflicts.  And I didn't properly retest.

Fix that, but try to make things less error prone.  Add a union to
represent the possible owner kinds for a target_section.  Trying to pass
a pointer to another type than those will not compile.

Change-Id: I600cab5ea0408ccc5638467b760768161ca3036c

gdb/progspace.c

@@ -207,10 +207,11 @@ program_space::exec_close ()
     {
       /* Removing target sections may close the exec_ops target.
 	 Clear ebfd before doing so to prevent recursion.  */
+      bfd *saved_ebfd = ebfd.get ();
       ebfd.reset (nullptr);
       ebfd_mtime = 0;
 
-      remove_target_sections (&ebfd);
+      remove_target_sections (saved_ebfd);
 
       exec_filename.reset (nullptr);
     }