[Spike] cvxif agent features

Created by: jfauchon

This branch contains a fix and a feature for the cvxif agent in spike:

• In the spec the custom instruction CUS_ADD has func7 equal to 0. In spike, their was no test to check that and so it decoded illegal instruction as CUS_ADD. Now it will raise illegal instruction.
• A new type of exception was created for all exceptions unknown by the cva6. This exception writes 0 in mtval, the cause in mcause and ends the test.

Signed-off-by: Jules FAUCHON jfauchon@thalesgroup.com