x86: Allow MCA messages to be rate-limited

Always print the first 50 messages of each type. After that, optionally
rate-limit the messages. This provides a way to limit the overhead of
processing excessive messages without suppressing the first few of any
type.

As part of this change, we are switching from direct printf() calls to
collecting data in an sbuf(9). In POLLED mode (run from a task queue),
we dynamically allocate the buffer. In the other modes (which are
likely called from a hardware interrupt), we use a buffer allocated
from the BSS segment and guarded by a lock. In normal operation,
most calls to mca_log() should come from the POLLED mode, so there
should be no contention for the new lock. If there is an interrupt
storm which exceeds the capacity of the free list, there will be new
contention for this lock; however, overall lock contention should
still be lower than it was prior to e770e32aa3a0, when the mca_lock
was almost always held for the entirety of the mca_log() call.

This commit is partly based on a patch proposed by Loic Prylli
<lprylli@netflix.com>.

Reviewed by: markj, glebius (previous version)
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D52946