-gcc version 14.2.0 (Debian 14.2.0-17) +gcc version 14.2.0 (Debian 14.2.0-16)
Note: gcc-14 in trixie is still at version 14.2.0-16, so you are still comparing the good build you made in unstable with the build made natively on your trixie system. As a result, the diff has a lot of unwanted noise. Either upgrade your system to unstable and compare good and bad builds both made in unstable, or build it in a trixie chroot to compare good and bad builds both made in trixie. Also, use deborphan from bookworm to remove any package which you don't really need, including binutils-gold (I don't think it's related but who knows). If it still fails, you could start adding packages to your chroot which you know they are also installed in your native system until the build also fails in the chroot. If there is an undeclared build-conflict, that would be the way to tell.
I guess my complier / building knowledge is more than exceeded here,> so sorry, that I cannot be of any help further (but of course, I can run further commands if necessary).
If everything else fails, consider reinstalling your testing system from scratch. At some point, doing that will be less time-consuming than debugging the problem. Your system seems to be messed up. Thanks.
