clarify mtval2/stval2/vstval2 handling

[tariqkurd-repo]

they need to be updated on all exceptions, not just CHERI ones

[arichardson]

Just to clarify that I understand the intent here: You are requiring it to be written to zero for non-CHERI traps?

Is that actually needed, or can we just keep a stale value here for a non-CHERI fault?

[jrtc27]

H has:

When a guest-page-fault trap is taken into HS-mode, htval is written with either zero or the guest
physical address that faulted, shifted right by 2 bits. For other traps, htval is set to zero, but a
future standard or extension may redefine htval’s setting for other traps.

When a guest-page-fault trap is taken into M-mode, mtval2 is written with either zero or the guest
physical address that faulted, shifted right by 2 bits. For other traps, mtval2 is set to zero, but a
future standard or extension may redefine mtval2’s setting for other traps.

The point is that, by writing 0 now, you can detect non-zero values should it ever start to be used for another existing trap.

[chrehrhardt]

Two instances where vstval should be vstval2. Otherwise LGTM.