cpu: Add Skylake-Server-IBRS CPU model
    
    This is a variant of Skylake-Server with indirect branch prediction
    protection. The only difference between Skylake-Server and
    Skylake-Server-IBRS is the added "spec-ctrl" feature.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>