x86 MCA: introduce MCA hooks for different vendor implementations

This is needed for AMD SMCA processors, as SMCA uses different
MSR address for access MCA banks.

Use IA32 specific msr_ops as defualt, and use SMCA-specific msr_ops
when on an SMCA-enabled processor

Submitted by: chandu from amd dot com
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D18055