s390: detect etoken facility

s390: detect etoken facility

CVE-2017-5715 (Spectre v2 s390x)

Detect and report the etoken facility. With spectre_v2=auto or
CONFIG_EXPOLINE_AUTO=y automatically disable expolines and use
the full branch prediction mode for the kernel.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
(cherry picked from commit aeaf7002a76c8da60c0f503badcbddc07650678c)
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>