i386: Add EPYC-IBPB CPU model

EPYC-IBPB is a copy of the EPYC CPU model with
just CPUID_8000_0008_EBX_IBPB added.

Cc: Jiri Denemark <jdenemar@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180109154519.25634-7-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>