bug#46965: 28.0.50; unexec does not build (or work)

[Pip Cet]

On this Debian-based x86_64-pc-linux-gnu system, configuring with
--with-dumping=unexec results in various errors:

1. a compilation error in pdumper.c. Easy to fix, my fault, but then there's ...
2. a segfault when running the dumped image. The problem is we can't
use glibc's malloc when running a dumped image, so we need to force
use of hybrid malloc. But then...
3. gnulib no longer builds with hybrid malloc. To do so requires some
massaging of the gnulib files, but then...
4. gnulib redirects free() to always point to the glibc free(), even
when Emacs wants to override it for hybrid malloc. To fix this, we
need to rerun gnulib-tool.

(1) tells me it's very unlikely anyone but me has tried to build
unexec Emacs lately. The other bugs tell me that it's getting rapidly
more difficult to support unexec build.

But unexec does provide performance advantages over pdumper, and even
over undumped builds.

So, do we want to keep it as an option? RMS said so in

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36649;msg=77

So I'm opening this bug for discussion, and to post the patches I need
to make unexec work again.