Do not call hw_mds_recalculate() from initializecpu().

If MDS mitigation is enabled by the tunable but MDS microcode is not
early-loaded, software mitigation is selected. This causes
initializecpu() to try to allocate memory which makes boot process
very unhappy.

Create SYSINIT that runs sufficiently late to succeed.

Reported by: naddy
PR: 237968
Sponsored by: The FreeBSD Foundation
MFC after: 1 week