x86: Reduce amount of time the MCA lock is held while emitting records

The MCA spin lock is acquired in the hardware interrupt context to
record MCA messages. It is also acquired by a task handler to emit
those messages.

Reduce the amount of time the task handler holds the lock to reduce
the maximum amount of time the hardware interrupt handler may need to
spin on the lock.

Reviewed by: glebius, markj
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D52938