MCA: add AMD Error Thresholding support

Currently the feature is implemented only for a subset of errors
reported via Bank 4. The subset includes only DRAM-related errors.

The new code builds upon and reuses the Intel CMC (Correctable MCE
Counters) support code. However, the AMD feature is quite different
and, unfortunately, much less regular.

For references please see AMD BKDGs for models 10h - 16h.
Specifically, see MSR0000_0413 NB Machine Check Misc (Thresholding)
Register (MC4_MISC0).
http://developer.amd.com/resources/developer-guides-manuals/

Reviewed by: jhb
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D9613