target/i386: add clflushopt to "Skylake-Server" cpu model

CPUID_7_0_EBX_CLFLUSHOPT is missed in current "Skylake-Server" cpu
model. Add it to "Skylake-Server" cpu model on pc-i440fx-2.12 and
pc-q35-2.12. Keep it disabled in "Skylake-Server" cpu model on older
machine types.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20171219033730.12748-3-haozhong.zhang@intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>