Add debug logging to new_thread about GC_threads hash table collisions

Add debug logging to new_thread about GC_threads hash table collisions

* pthread_support.c [DEBUG_THREADS] (GC_new_thread): Call GC_log_printf
(with the appropriate message) if a GC_threads[] entry already exists
at the given hv index (excluding the detached threads those id is
already reused).
* win32_threads.c [DEBUG_THREADS] (GC_new_thread): Call GC_log_printf
to report that a new thread is created; call GC_log_printf (with the
appropriate message) if a GC_threads[] entry already exists at the
given hv index; remove useless comment about the result of
GC_INTERNAL_MALLOC.