Adjust logged messages in start_mark_threads and GC_thr_init

Adjust logged messages in start_mark_threads and GC_thr_init

* pthread_support.c (start_mark_threads): Move GC_COND_LOG_PRINTF call
to the function end (i.e., call pthread_attr_destroy first).
* pthread_support.c (GC_thr_init): Do not log number of marker threads
since start_mark_threads logs number of started mark helper threads
(only if PARALLEL_MARK).
* win32_threads.c (start_mark_threads): Update comment; move logging
of started mark helper threads from GC_thr_init().
* win32_threads.c (GC_thr_init): Log corresponding message if parallel
marking is disabled (similar to that in pthread_support.c, only if
PARALLEL_MARK).