Enable x86 retpoline options by default

[Steve Beattie]

For Ubuntu 18.04 LTS, the x86 retpoline options (-mindirect-branch=thunk and -mfunction-return=thunk) should be enabled by default.

For problematic packages, -mindirect-branch=keep and -mfunction-return=keep can be used to disable retpolines.

[Dimitri John Ledkov (xnox)]

A dpkg-buildflags integration could be nice. Such that e.g. DEB_BUILD_MAINT_OPTIONS=hardening=-retpoline works.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Matthias Klose (doko)]

afaics the decision was not to rebuild user space with these patches enabled.