x86: support sending non-fatal MCA messages to syslog

Currently, all MCA messages are logged to the console. However, this
can have performance implications which can turn a storm of correctable
errors into a disruptive event.

Provide an option to direct non-fatal messages to syslog. Note that
we will always log to the console for fatal errors.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D12278