Change type of THREAD_TABLE_INDEX result to int in win32_threads.c

Change type of THREAD_TABLE_INDEX result to int in win32_threads.c
(code refactoring)

* win32_threads.c (THREAD_TABLE_INDEX): Cast result to int; remove cast
of argument to word type; add comment.
* win32_threads.c (GC_new_thread, GC_delete_gc_thread_no_free,
GC_delete_thread): Change type of hv local variable from word to int.
* win32_threads.c (GC_lookup_thread_inner): Remove hv local variable;
remove "register" keyword for p local variable.
* win32_threads.c (GC_lookup_pthread): Change type of hv_guess local
variable from word to int.