UBUNTU: [Packaging] retpoline -- add safe usage hint support

UBUNTU: [Packaging] retpoline -- add safe usage hint support

BugLink: http://bugs.launchpad.net/bugs/1758856
Use the upstream retpoline safe hinting support to clear out known
safe retpoline sequences from those detected.  At the same time
switch to extracting the indirect sequences and associated hints
to .o generation time.  This allows it to be run on the cache hot
data and to be run in parallel on the builders.

Signed-off-by: Andy Whitcroft <apw@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>