Specify an Exception Trigger (#266)

* Specify an Exception Trigger

I tried to get all the sources from the Syntacore proposal in, but did
not add the various timing options. We do get interrupt sources now as
well as the exception sources.

* Redo etrigger.

Now we support every source that's explicitly mentioned in the mcause
definition in the privspec.

Also make a separate table for \Faction instead of repeating its
definition over and over.

* Allow more interrupts/causes to be triggered on.

Clarified use of \Faction a bit as well.

* Incorporate review feedback.

* Attempt to clarify u/s/m

* Break exception trigger into two triggers.

This avoids using tdata3, which helps with certain implementations where
trigger types are writable.

* Rebuild pdf.