GitLab

Extend the CI checkstyle workflow to perform the library ABI
checks in the master branch.  The intent is not to prevent any
ABI changes but to detect them immediately so when they're
made it's done intentionally.

When the changing the ABI the `make storeabi` target can be
used to generate a new .abi file which can be included with
the commit.  This depends on the libabigail utility which is
available from the majority of distribution package managers.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11287