[SRU trusty/xenial/artful/bionic/unstable] LP#1758856/LP#1760876 -- retpoline hints cleanups

[Andy Whitcroft]

We have a few minor issues with the existing retpoline hints code.
Firstly if retpoline is disabled we may not have any extracted files in
the headers.  We also now expect the retpoline files to be empty.
Finally to allow us to build out of tree modules we need the extractor
in the headers package.

Patches for these as needed in each series follow.  Where retpoline-hints
has already been applied they are annotated against master-next, where
it has not they are annotated retpoline-hints.

Proposing for SRU to all series in which we apply the retpoline hints
fixes.

-apw