Replace non-API occurrences of GC_word to word

Replace non-API occurrences of GC_word to word
(code refactoring)

* finalize.c (last_finalizer_notification): Use "word" type instead of
GC_word.
* include/private/dbg_mlc.h (HIDE_BACK_PTR): Likewise.
* include/private/gc_priv.h (WARN): Likewise.
* include/private/gcconfig.h (POINTER_MASK): Likewise.
* mark.c (GC_push_marked): Likewise.
* misc.c (GC_init, GC_set_max_retries): Likewise.
* os_dep.c (GC_unmap, GC_remap, GC_unmap_gap): Likewise.
* typd_mlc.c (GC_add_ext_descriptor): Likewise.
* include/private/gcconfig.h (GC_amiga_get_mem, ps3_get_mem): Rename
argument to bytes.