CVE-2017-5715 (Spectre v2 Intel)

Set IBRS upon kernel entrance via syscall and interrupts. Clear it upon exit.
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit bb6c1a01e82fb0eb14d1229fd71a99ed285d330d)
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>