Use GC_WORD_MAX macro across all C source files

Use GC_WORD_MAX macro across all C source files
(code refactoring)

* alloc.c (GC_collect_at_heapsize, GC_compute_heap_usage_percent,
GC_add_to_heap, GC_expand_hp_inner): Use GC_WORD_MAX instead of
(word)-1.
* dyn_load.c [DATASTART_IS_FUNC]
(GC_register_dynamic_libraries_dl_iterate_phdr): Likewise.
* finalize.c [KEEP_BACK_PTRS] (GC_notify_or_invoke_finalizers):
Likewise.
* headers.c (GC_install_counts): Likewise.
* include/private/thread_local_alloc.h [GC_GCJ_SUPPORT] (ERROR_FL):
Likewise.
* os_dep.c [NEED_FIND_LIMIT || USE_PROC_FOR_LIBRARIES] (GC_find_limit):
Likewise.
* os_dep.c [MPROTECT_VDB && DARWIN] (GC_mprotect_thread): Likewise.
* pthread_support.c [PARALLEL_MARK] (GC_mark_thread): Likewise.
* win32_threads.c (ADDR_LIMIT): Likewise.
* win32_threads.c [PARALLEL_MARK] (GC_mark_thread): Likewise.
* alloc.c (GC_least_plausible_heap_addr): Use GC_WORD_MAX instead of
ONES macro.
* include/private/gc_priv.h [STACK_GROWS_DOWN] (MAKE_COOLER): Likewise.
* mark.c (GC_set_hdr_marks): Likewise.
* alloc.c (GC_WORD_MAX): Remove definition (in this file).
* headers.c (GC_install_counts): Reformat code.
* include/private/gc_priv.h (ONES): Rename to GC_WORD_MAX.