Use GC_log_printf for logging instead of GC_[err_]printf

Use GC_log_printf for logging instead of GC_[err_]printf

* backgraph.c (GC_print_back_graph_stats): Use GC_err_printf instead
of GC_printf (since invoked GC_print_heap_obj uses GC_err_printf).
* darwin_stop_world.c (GC_FindTopOfStack): Replace GC_printf with
GC_log_printf (inside DEBUG_THREADS blocks).
* include/private/gc_priv.h (GC_printf, GC_log_printf): Add comment.
* malloc.c (GC_free): Replace GC_err_printf with GC_log_printf (inside
LOG_ALLOCS block).
* thread_local_alloc.c (GC_malloc): Likewise.