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

[jtl]

Summary

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.

[markj]

sys/x86/x86/mca.c

1047

It'd might be marginally cheaper to concatenate tmplist onto the end of the record list, and then remove excess elements from the front of the record list. But I'm not sure it's worth the complexity.