abidiff returns 4 when the ABI has changed but not necessarily in an
incompatible way, which is something we want to allow as the new
functions may be added to the stable branch.
Add the ABI files generated by libabigail abidw tool (note that
--no-corpus-path --no-comp-dir-path options were used to avoid having
system-specific object file paths in these files and the source file
paths were edited to remove the system-specific prefix from them too)
and a script comparing the current ABI with these files.
Also update the documentation to describe using libabigail tools as
abi-compliance-checker doesn't seem to work under the recent systems any
longer: it gives some warnings and doesn't detect ABI-incompatible
changes.
Closes#22634.
