Eliminate more 'scope of variable can be reduced' cppcheck style warnings

Eliminate more 'scope of variable can be reduced' cppcheck style warnings

* allchblk.c (GC_compute_large_free_bytes, GC_print_hblkfreelist,
free_list_index_of, GC_dump_regions, GC_add_to_fl, GC_unmap_old,
GC_allochblk_nth): Move local variable declaration to the inner scope
where the variable is actually used.
* alloc.c (GC_maybe_gc, GC_collect_a_little_inner, GC_set_fl_marks,
GC_clear_fl_marks): Likewise.
* dyn_load.c (GC_register_dynamic_libraries): Likewise.
* finalize.c (GC_ignore_self_finalize_mark_proc,
GC_register_finalizer_inner, GC_dump_finalization_links,
GC_dump_finalization, GC_make_disappearing_links_disappear,
GC_remove_dangling_disappearing_links, GC_invoke_finalizers): Likewise.
* headers.c (GC_install_counts): Likewise.
* malloc.c (GC_malloc_kind_global, GC_generic_malloc_uncollectable,
GC_free): Likewise.
* mark.c (GC_push_all_eager, GC_push_marked1, GC_push_marked2,
GC_push_marked4): Likewise.
* mark_rts.c (GC_next_exclusion, GC_exclude_static_roots_inner,
GC_push_conditional_with_exclusions): Likewise.
* misc.c (GC_base, GC_parse_mem_size_arg, GC_write): Likewise.
* os_dep.c (GC_repeat_read, GC_get_maps, GC_least_described_address,
GC_register_root_section, GC_register_data_segments): Likewise.
* reclaim.c (GC_reclaim_block, GC_start_reclaim): Likewise.
* finalize.c (ITERATE_DL_HASHTBL_BEGIN): Declare prev_dl local variable.