retpoline: Add retpoline tag to VERMAGIC

Add a marker for retpoline to the module VERMAGIC. This catches
the case when a non RETPOLINE compiled module gets loaded into
a retpoline kernel, making it insecure.

It doesn't handle the case when retpoline has been runtime disabled.
Even in this case the match of the retcompile status will be enforced.
This implies that even with retpoline run time disabled all modules
loaded need to be recompiled.

This supersedes an earlier patch that did the same checking using
a new module tag (so it's really a v3)

Signed-off-by: Andi Kleen <ak@linux.intel.com>