Implement machine-mode

[stephank]

Currently, we just implement stuff from the user-level instruction set (spec volume 1), but according to volume 2, machine-mode is also required. I remember the test suite also tried to manipulate machine-mode CSRs, and we currently hack around this.

I'm working on implementing this, because I'd like to have interrupts. The idea is to have just machine-mode for now, and not implement supervisor or user privilege levels. The spec allows this.

But it's also something of a breaking change, I think. Mostly because exceptions no longer go to the environment, but the machine can handle them itself. I'm not sure if it can be an optional thing. Is it something we want to try make optional?

[stephank]

Not working on this anymore.