Move the mds, irbs, and ssb mitigation knobs into machdep.mitigations.
They're in both the old and new places in HEAD for the moment for
discussion and transition. The old locations will be garbage collected
in 4 weeks. MFCs to 12 an 11 will keep the old and new for transition
purposes.

Reviewed by: kib
MFC after: 4 weeks
Sponsored by: Intel
Differential Revision: https://reviews.freebsd.org/D22590