powerpc/64s: Wire up cpu_show_spectre_v2()

powerpc/64s: Wire up cpu_show_spectre_v2()

BugLink: https://bugs.launchpad.net/bugs/1760099
CVE-2017-5715

Add a definition for cpu_show_spectre_v2() to override the generic
version. This has several permuations, though in practice some may not
occur we cater for any combination.

The most verbose is:

  Mitigation: Indirect branch serialisation (kernel only), Indirect
  branch cache disabled, ori31 speculation barrier enabled

We don't treat the ori31 speculation barrier as a mitigation on its
own, because it has to be *used* by code in order to be a mitigation
and we don't know if userspace is doing that. So if that's all we see
we say:

  Vulnerable, ori31 speculation barrier enabled

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit d6fbe1c55c55c6937cbea3531af7da84ab7473c3 linux-next)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>