Add support for the "E" extension

[coastalwhite]

This PR proposes a small changes to increase support for the "E" extension. This does not necessarily mean that everything already works for the "E" extension, this is just a step in that direction.

[romancardenas]

Thanks for your contribution! LGTM, let's wait for the CI to finish.

I guess you also need to change the trap handler routine. Is it OK if I leave the PR open as an RFC in which you can keep working on providing support to the RISCV E?

[coastalwhite]

Yeah, that is 100% okay. I will work on this a bit further. I’ll let you know when it’s ready.

[romancardenas]

This repository has been moved to riscv. Please, re-open the PR in the new repo.