x86: Keep cumulative MCA statistics in the kernel

Keeping cumulative MCA statistics in the kernel provides a way for
users to get an accurate count of various kinds of errors reported
by the CPU.

After ca8929d2a3e9b9df31d2e487377f99d7c39aa01d, it is possible that
the kernel will drop the record of some MCA interrupts. Moreover, this
provides a cheaper interface to obtain statistics if that is the only
reason a user is processing MCA logs.

Reviewed by: markj
Sponsored by: Netflix
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52709