Replace var-args GC_noop with fixed-args one

Replace var-args GC_noop with fixed-args one
(to suppress Clang warning that arguments were dropped)

* alloc.c (GC_stopped_mark, GC_try_to_collect_general): Use GC_noop6
instead of var-args GC_noop.
* include/private/gc_priv.h (GC_noop): Replace var-args (and
platform-specific) prototype with GC_noop6 one.
* mark.c (GC_noop): Replace with GC_noop6 (with all arguments
ignored); refine comment.