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