ci: enable v21 ABI checks

ci: enable v21 ABI checks

v21 ABI will be maintained until v21.11.

Let's use the latest released libabigail 1.8.

In GitHub Actions, libabigail binaries and the ABI reference are stored
in two shared caches as all branches can use the same.

While at it, we can reproduce changes from the commit 0b8086ce3fe7
("devtools: remove useless files from ABI reference").
This will save some space in the CI caches.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Aaron Conole <aconole@redhat.com>