BugLink: https://bugs.launchpad.net/bugs/1791080

Expose indirect_branch_prediction_barrier() for use in subsequent patches.
Our approach for IBRS/IBPB is still based on the Intel v4 patchset and I
did not want to completely switch that in this upstream stable set. For
that reason I skipped most changes from "x86/speculation: Add basic IBPB
(Indirect Branch Prediction Barrier) support" from the 4.4.144 update.

Only indirect_branch_prediction_barrier() I picked up because it will
later be used for an update to "x86/speculation: Use Indirect Branch
Prediction Barrier in context switch".
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>