Add GC_reachable_here after GC_END_STUBBORN_CHANGE in cords

Add GC_reachable_here after GC_END_STUBBORN_CHANGE in cords
(fix of commit e12e820f3)

* cord/cordbscs.c (CORD_cat_char_star, CORD_cat, CORD_from_fn_inner,
CORD_substr_closure): Call GC_END_STUBBORN_CHANGE() instead of
GC_end_stubborn_change().
* cord/cordxtra.c (refill_cache): Likewise.
* cord/tests/de.c (prune_map, add_map, add_hist, replace_line,
generic_init): Likewise.
* cord/cordbscs.c (CORD_cat_char_star, CORD_cat): Mark x and y as
reachable after GC_END_STUBBORN_CHANGE(result).
* cord/cordbscs.c (CORD_from_fn_inner): Mark client_data as reachable
after GC_END_STUBBORN_CHANGE(result).
* cord/cordbscs.c (CORD_substr_closure): Mark x as reachable after
GC_END_STUBBORN_CHANGE(sa).
* cord/tests/de.c (prune_map): Mark saved map->previous->previous as
reachable after GC_END_STUBBORN_CHANGE(map).
* cord/tests/de.c (add_map): Mark saved current_map as reachable after
GC_END_STUBBORN_CHANGE(new_map).
* cord/tests/de.c (replace_line): Mark s as reachable after
GC_END_STUBBORN_CHANGE(screen+i).