x86: Implement MWAIT support for stopping a CPU

IPI_STOP is used after panic or when ddb is entered manually.  MONITOR/
MWAIT allows CPUs that support the feature to sleep in a low power way
instead of spinning.  Something similar is already used at idle.

It is perhaps especially useful in oversubscribed VM environments, and is
safe to use even if the panic/ddb thread is not the BSP.  (Except in the
presence of MWAIT errata, which are detected automatically on platforms with
known wakeup problems.)

It can be tuned/sysctled with "machdep.stop_mwait," which defaults to 0
(off).  This commit also introduces the tunable
"machdep.mwait_cpustop_broken," which defaults to 0, unless the CPU has
known errata, but may be set to "1" in loader.conf to signal that mwait
wakeup is broken on CPUs FreeBSD does not yet know about.

Unfortunately, Bhyve doesn't yet support MONITOR extensions, so this doesn't
help bhyve hypervisors running FreeBSD guests.

Submitted by:   Anton Rang <rang AT acm.org> (earlier version)
Reviewed by:	kib
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D20135

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