last_marked array is now ifdef'ed away

[Eli Zaretskii]

Commit 7a8798de95a5 (from Apr 4 2022) made the last_marked[] array and
the related machinery of tracing the marked objects #ifdef'ed away by
default:

  +/* Whether to remember a few of the last marked values for debugging.  */
  +#define GC_REMEMBER_LAST_MARKED 0
  +
  +#if GC_REMEMBER_LAST_MARKED
   enum { LAST_MARKED_SIZE = 1 << 9 }; /* Must be a power of 2.  */
   Lisp_Object last_marked[LAST_MARKED_SIZE] EXTERNALLY_VISIBLE;
   static int last_marked_index;
  +#endif
  [...]
  -  last_marked[last_marked_index++] = obj;
  -  last_marked_index &= LAST_MARKED_SIZE - 1;
  +#if GC_REMEMBER_LAST_MARKED
  +      last_marked[last_marked_index++] = obj;
  +      last_marked_index &= LAST_MARKED_SIZE - 1;
  +#endif

I don't remember this aspect being discussed, and the commit log
message doesn't even mention the change, let alone provides a
rationale for it.

Mattias, why was this done?  Are the changes you introduced in that
changeset somehow incompatible with the last_marked[] facility?  If
not, I think we should make this again compiled-in by default, because
IME it is a valuable means of debugging GC problems.