Explore using Abigail as an extra test to track ABI changes

Current dpkg-gensymbols has several limitations, see thread https://lists.debian.org/debian-devel/2025/09/msg00199.html.

E.g. @rouca has been looking into alternatives. In Lucas DebConf25 talk on distro-wide testing the audience members seemed to think that https://sourceware.org/libabigail/ is better than https://lvc.github.io/abi-compliance-checker/

There is also Bug#944459 about integrating Abigail in Britney. We should look into providing Abigail as an easy-to-use optional job in Salsa CI.

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