In Ubuntu, we have runtime control for enabling/disabling IBRS via the
commandline ("noibrs") and through the proc interface
/proc/sys/kernel/ibrs_enabled. This commit simplifies the current
(probably broken) implementation by merging it with all the IBRS-related
upstream changes from previous commits.

What we have now is the upstream implementation for detecting the presence
of IBRS support. This commit adds a global state variable 'ibrs_enabled'
which is set to 1 if the CPU supports IBRS but can be overridden via the
commandline "noibrs" switch or by writting 0, 1 or 2 to
/proc/sys/kernel/ibrs_enabled at runtime.

Note that the runtime controls are disabled if the CPU runs in Enhanced
IBRS mode.

CVE-2017-5715
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>