Reinstall recent GC-related changes

Reinstall recent GC-related changes

The report that they broke macOS was a false alarm, as the
previous commit was also broken (Bug#43152#62).
* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding):
Count only pointers that point to a struct component,
or are a tagged pointer to the start of the struct.
Exception: for non-bool-vector pseudovectors,
count any pointer past the header, since it’s too much
of a pain to write code for every pseudovector.
(live_float_holding, live_vector_pointer):
New functions, which are similar about counting pointers.
(live_float_p, live_large_vector_holding)
(live_small_vector_pointer, mark_maybe_pointer): Use them.
(mark_maybe_object, mark_maybe_objects): Remove,
and remove all callers; mark_maybe_pointer now suffices.
(mark_objects): New function.
* src/alloc.c (mark_vectorlike, mark_face_cache):
* src/eval.c (mark_specpdl):
* src/fringe.c (mark_fringe_data):
* src/keyboard.c (mark_kboards):
Simplify by using mark_objects.
* src/lisp.h (SAFE_ALLOCA_LISP_EXTRA):
Clear any Lisp_Object arrays large enough to not fit into the stack,
so that GC need not worry about whether they contain objects.