cpu: Add Broadwell-IBRS CPU model

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>