CVE-2018-3639 (x86)

Now that we have generic, vendor-agnostic support of IBPB/IBRS feature
detection in common code, move the code to enable/disable it from the
vendor-specifc init_<vendor> functions to the common code.

Except for the AMD special case where we need to write an MSR on every CPU.
Keep that in init_amd() which runs on every CPU, whereas the common code
mentioned above only runs once when CPU 0 is being onlined.
Signed-off-by: Juerg Haefliger <juergh@canonical.com>