Introduce an empty arch_smt_update() stub to be used in subsequent x86
speculation code.

It's basically commit 53c613fe ("x86/speculation: Enable
cross-hyperthread spectre v2 STIBP mitigation") without the STIBP-specific
pieces.

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>