Add ABI snapshot

Add a snapshot of the current ABI using libabigail-1.7-2. The
included ABI passes make checkabi for CentOS 7, Fedora 33,
Debian 10, and Ubuntu 20.04. This covers a fairly wide range
of glibc, gcc, and libabigail versions plus other changes which
are platform specific.

Reviewed-by: Antonio Russo <aerusso@aerusso.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11144