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>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>