Add 'const' qualifier to pointer argument of GC_clear/set_mark_bit,

Add 'const' qualifier to pointer argument of GC_clear/set_mark_bit,
GC_finalized_malloc, GC_general_register_disappearing_link

* dbg_mlc.c (GC_debug_change_stubborn, GC_debug_end_stubborn_change):
Make argument pointer const.
* finalize.c (GC_general_register_disappearing_link): Likewise.
* fnlz_mlc.c (GC_core_finalized_malloc, GC_finalized_malloc): Likewise.
* include/gc.h (GC_change_stubborn, GC_end_stubborn_change,
GC_debug_change_stubborn, GC_debug_end_stubborn_change,
GC_general_register_disappearing_link): Likewise.
* include/gc_disclaim.h (GC_finalized_malloc): Likewise.
* include/gc_mark.h (GC_clear_mark_bit, GC_set_mark_bit): Likewise.
* mark.c (GC_set_mark_bit, GC_clear_mark_bit): Likewise.
* stubborn.c (GC_end_stubborn_change, GC_change_stubborn): Likewise.
* finalize.c (GC_general_register_disappearing_link): Remove redundant
pointer type casts.
* tests/test.c (cons, print_int_list, mktree): Likewise.
* include/gc.h (GC_GENERAL_REGISTER_DISAPPEARING_LINK): Add cast
removing potential 'const' qualifier.