cpu: Add Broadwell-IBRS CPU model
    
    This is a variant of Broadwell with indirect branch prediction
    protection. The only difference between Broadwell and Broadwell-IBRS is
    the added "spec-ctrl" feature.
    
    The Broadwell-IBRS model in QEMU is a bit different since Broadwell got
    several additional features since we added it in cpu_map.xml:
        abm, arat, f16c, rdrand, vme, xsaveopt
    
    Adding them only to the -IBRS variant would confuse our CPU detection
    code.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>