I was a bit curios what is going on there. It turnes out, at least as I
understand, that the errors were not real errors, in most cases, so it seems
*almost* legit to brush them under carpet.

I printed out errors and what I saw were endless

"Write error to standard output: no error"

messages when native compiler run its batch jobs (amongst other).

Seems like close_stream in ucrt runtime does not return real errno but something
else.

Just checking if errno was set reduced almost everything; and I found that
comment in posix_close aboutt EINTR.

However; I am still not able to build emacs wihtout trolling away exit on
failure, because temacs fails. I get this error:

cp -f temacs.exe bootstrap-emacs.exe
rm -f bootstrap-emacs.pdmp
./temacs --batch  -l loadup --temacs=pbootstrap \
        --bin-dest /ucrt64/bin/ --eln-dest /ucrt64/lib/emacs/30.0.50/
C:\Users\arthu\repos\emsrc\ucrt-02-21\src\temacs.exe: Write error to standard output: No such file or directory
make[2]: *** [Makefile:1014: bootstrap-emacs.pdmp] Error 1

Question is which file or directory? Dump file? Bad path? Bad encoding?
Something in loadup.el or elsewhere?

If I comment away exit on failure as they do in mingw patch, than everything
builds and seemnigly works. However I have experienced one crash where system
killed Emacs, similar as those I have seen with the version from gnu ftp
(29.2_1).

How do I debug temacs bootstrap?