sim:epiphany: Clean up OOB handling

Remove some OOB events that really weren't out of band events. Handle them
directly on the register access instead. Currently the only events that
classify as OOB are those that can change program flow (PC).
sim: epiphany: epiphany-elf-sim: Use relative path to find dummy binary
sim: epiphany: Fix check for epiphany-elf-as

Fix check for epiphany-elf-as so that the dummy program for the
simulator frontend gets built if possible.