Decode AMD RAS Capabilities and enable MCA thresholding on 17h (two commits)

[cem]

Summary

Store AMD RAS Capabilities cpuid value and name flags

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.

[cem]

Fix a braino.

[truckman]

Builds fine and doesn't cause any obvious breakage on my Ryzen. This feature does appear to be detected properly.