cpu_x86: Copy CPU signature from ancestor

cpu_x86: Copy CPU signature from ancestor

When specifying a new CPU model in cpu_map.xml as an extension to an
existing model, we forgot to copy the signature (family + model) from
the original CPU model.

We don't use this way of specifying CPU models, but it's still supported
and it becomes useful when someone wants to quickly hack up a CPU model
for testing or when creating additional variants of existing models to
help with fixing some spectral issues.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>