x86 MCA: Enable AMD thresholding support on 17h

17h supports MCA thresholding in the same way as 16h and earlier.
Supposedly a ScalableMca feature bit in CPUID 8000_0007:EBX must be set, but
that was not true for earlier models, so be careful about relying on it.

While here, document a missing bit in LS MCA MISC0.

Reviewed by: truckman
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12237