docs: add a table showing x86-64 ABI compatibility levels

It is useful to know which CPUs satisfy each x86-64 ABI
compatibility level, when dealing with guest OS that require
something newer than the baseline ABI.

These ABI levels are defined in:

  https://gitlab.com/x86-psABIs/x86-64-ABI/



and supported by GCC, Clang, glibc and more.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210607135843.196595-2-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>