Delegate exceptions based on medeleg

[mtvec]

This PR implements exception delegation based on medeleg as well as a UI setting to either delegate all or no exceptions.

This fixes #96

[TheThirdOne]

Thanks for taking a stab at implementing this.

Some minor problems:

• medeleg doesn't have any hint text.
• trying to read medeleg should throw an illegal instruction exception
  • This might merit a new type of register that is only viewable/modifiable from GUI / internal code.
  • It might be interesting to have medeleg just not reset its value and remove the setting.
• ecall (only if there is a valid system call) and ebreak bypass the normal exception trap handler.

Additionally I think the handling of exception traps is incorrect in two ways:

• If utvec = 0, the normal error message should not be used. Instead it should either enter a tight loop or output a message about the trap handler address being invalid.
• It should not set UPIE = 1. Instead it should set UPIE = UIE.

The csr test would be unable to pass if the ecall issue were fixed. Neither default value of medeleg would let it detect invalid CSRs and actually signal completion.

[TheThirdOne]

It seems like no progress is currently being made on this. I plan to address the problems myself.

I plan to make a new class of register that is only read/writeable internally and persists its value across assembly; medeleg will be an instance of this. I will also remove the setting for delegating everything; if you are using the GUI and want something delegated, just set the value manually once per startup. Support for changing medeleg from the CLI will be left for a later time. The automated tests can set medeleg to whatever is needed to correctly test behavior. ecall and ebreak correctness will also be left for a later time. I plan to fix the exception handling issues and hint text issues.

[mtvec]

It seems like no progress is currently being made on this. I plan to address the problems myself.

Apologies, I somehow completely lost track of this and only noticed your responses now. Since you are going to address this yourself, I will close this PR.