When a machine check event is detected (including a AMD RevF threshold
overflow event) allow to run a "trigger" program. This allows user space
to react to such events sooner.

The trigger is configured using a new trigger entry in the
machinecheck sysfs interface. It is currently shared between
all CPUs.

I also fixed the AMD threshold handler to run the machine
check polling code immediately to actually log any events
that might have caused the threshold interrupt.

Also added some documentation for the mce sysfs interface.
Signed-off-by: NAndi Kleen <ak@suse.de>