Add Exception element to RISC-V builder

[romancardenas]

As part of my work in this PR, I noticed that I forgot a mechanism to define custom exception sources. This PR should fix this.

[burrbull]

Please, avoid merging master branch. Use rebase instead.

[burrbull]

Is Exception structure same as Interrupt?
We shoud reuse it instead of creating a copy.

[romancardenas]

(Sorry, I used the sync button in GitHub)

I used copies so the tag ID is exception instead of interrupt. In RISC-V literature, it is common to see a clear separation between exceptions (synchronous events associated with an instruction) and interrupts (external event that occurs asynchronously to the current running thread). In any case, I don't have a strong opinion on this, so if you prefer, I can use interrupt elements instead of the current exception copy.