bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Pip Cet]

I believe this isn't the problem we're looking for, but it might be
related anyway.

I'm seeing this in the assembler source code for insdel.c produced
with the mingw cross compiler (i686-w64-mingw32-gcc-win32):

    movl    60(%esp), %eax
    movl    %eax, (%esp)
    movl    72(%esp), %eax
    movl    %eax, 4(%esp)
    call    _Fmarker_position
If I'm reading this correctly, it's of some concern for wide-int
builds: the two 32-bit halves of a Lisp_Object are stored
non-consecutively.

Our stack marking doesn't catch that; at least, it doesn't for
symbols, where the less-significant half isn't a valid pointer. For
pseudovectors, things should still work...

So I think we have a problem with such --wide-int builds in cases
where a stack temporary holds an unpinned uninterned symbol while GC
is called. Something like

(prog1
  (gensym)
  (garbage-collect))

might trigger it. No problem with gcc -m32 on GNU/Linux, for some reason.