[BUG] Incorrect TVAL cases in decoder

[PRugg-Cap]

Is there an existing CVA6 bug for this?

• [x] I have searched the existing bug issues

Bug Description

Further to https://github.com/openhwgroup/cva6/issues/3018, the following TVAL behaviours currently in CVA6 seem to be illegal from reading the spec.

• ECALL reports the instruction bits in TVAL: it should write zeros
• EBREAK reports the instruction bits in TVAL: it should report the instruction PC (I think that's the "faulting virtual address" in this case?)

These were found by fuzzing against Sail with TestRIG

If people agree, I'm happy to file a PR to fix these.