A few improvements of the l0dable runtime

I was looking at the l0dable CRT and found a few things which could be improved:

1. After main() exits, don't busy spin but return from the l0dable with epic_exit().
2. In case of an exception, don't lock up the device but also attempt returning with epic_exit(). Without this, the only way to recover would be a full reboot of the card10.
3. Remove some bloat by making all unused interrupt handlers weak references to the default handler.

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