Thank you. It finally works.

While testing I had another problem related to pdumper stuff getting 
used without being defined. So I wrote a trivial patch for it. But then 
I retested with a clean bootstrap and learned that it normally only 
gives a warning for it:

make[2]: Entering directory '/d/Emacs/configurations/master-unexec/lisp'
   ELC      /d/Emacs/source/repo/lisp/emacs-lisp/bytecomp.elc

In end of data:
D:/Emacs/source/repo/lisp/emacs-lisp/bytecomp.el:5314:39: Warning: the
     function `pdumper-stats' is not known to be defined.

Anyway, I've attached that patch, feel free to apply or not apply it as 
you see fit, since it only removes that one warning, and since unexec is 
deprecated anyway...