x86: Use control flow typechecking where possible

x86: Use control flow typechecking where possible

Now all indirect branch targets have been annotated, turn on typechecking to
catch issues in the future.

This extension isn't in a released version of GCC yet, so provide a container
to use with the extension included, and add it to CI.  RANDCONFIG is necessary
because some stubs for compiled-out subsystems are used as function pointer
targets.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>