Tag must-be-non-null arguments of GC API functions with the attribute

Tag must-be-non-null arguments of GC API functions with the attribute

* include/gc.h (GC_set_oom_fn, GC_strndup, GC_posix_memalign,
GC_change_stubborn, GC_end_stubborn_change, GC_size,
GC_try_to_collect, GC_set_stop_func, GC_debug_strndup,
GC_debug_change_stubborn, GC_debug_end_stubborn_change, GC_wcsdup,
GC_debug_wcsdup, GC_register_finalizer, GC_debug_register_finalizer,
GC_register_finalizer_ignore_self,
GC_debug_register_finalizer_ignore_self,
GC_register_finalizer_no_order, GC_debug_register_finalizer_no_order,
GC_register_finalizer_unreachable,
GC_debug_register_finalizer_unreachable,
GC_register_disappearing_link, GC_general_register_disappearing_link,
GC_move_disappearing_link, GC_set_warn_proc, GC_call_with_alloc_lock,
GC_call_with_stack_base, GC_register_my_thread, GC_do_blocking,
GC_call_with_gc_active, GC_get_stack_base, GC_pre_incr, GC_post_incr):
Specify 'non-null' attribute for the relevant function argument.
* include/gc_backptr.h (GC_get_back_ptr_info, GC_print_backtrace):
Likewise.
* include/gc_mark.h (GC_new_kind, GC_new_kind_inner, GC_is_marked,
GC_clear_mark_bit, GC_set_mark_bit): Likewise.
* include/gc_config_macros.h (GC_ATTR_NONNULL): New macro (support
'non-null' attribute for GCC v4+).