amd64: properly recalculate mitigations knobs after resume

[kib]

Summary

Revision r333125 AKA 986c4ca38772f72 forced clear cpu_stdext_feature3
on suspend, since at that time microcode update was not reloaded
early on resume. Then, revision 8405c63dadaf started re-reading
cpu_stdext_feature3 again. Since modern CPUs do not require mitigations
from the Skylake era, this went unnoticed for some time.

Keep zeroing cpu_stdext_feature3 on suspend, but re-read it in more
controlled way on resume after microcode is reloaded, and recalculate
active workarounds based on actual microcode capabilities.

Reported and tested by: romain

[markj]

I do not have commit 8405c63dadaf in my tree. Commit 050f5a8405c63 probably?

sys/amd64/acpica/acpi_wakeup.c

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