RVV compile option to enable decoding vector instructions

[kpgriesser]

This PR adds a compile option, RVV, to allow REV to be built to support the vector coprocessor. This PR is required for a forthcoming PR in the R2V2 repository that decouples that design from the REV source tree.

[leekillough]

We should compare / consolidate this with the REVVEC macro definition.

#ifdef REVVEC
#include "insns/RVVec.h"
#endif

[kpgriesser]

RVVec.h,. the vector instruction implementations, no longer exists in the rev source tree. The now independent coprocessor now includes this header so we can remove the #ifdef REVVEC code block. ( Tested with 'solo' branch in r2v2 repo )

[leekillough]

I was mainly speaking of the REVVEC vs RVV naming discrepancy -- consolidate one and retire the other, as well as integrate them into the build. grep REVVEC src/* include/* include/insns/* to make sure there aren't others.

I don't think we need the RevFeature V extension parsing to be enabled/disabled by an #ifdef. Like Zicntr, a runtime error can be printed if a V coprocessor is not available.

[kpgriesser]

There are no other REVVEC defines. If nobody has an issue with have the V extension parsing on by default ( I don't ) then we can just remove the RVV define as well and not require any special compile.

[leekillough]

I would prefer if we didn't have to build Rev differently. I think the parsing of the V extension can always be supported. If the V instructions are handled by the coprocessor during decode, and the coprocessor code has access to RevFeature (directly or indirectly -- we can always add more accessors if the current passkey mechanism is insufficient), then it can tell whether to decode V instructions or just ignore them and cause a normal unable to decode instruction error in Rev Core. Or if it wants to be more fancy, the coprocessor can decode V instructions as if the V extension is enabled, and then abort with a runtime error if the V feature is not really enabled, similar to how Zicntr works today.

[kpgriesser]

I agree. My last push removes the RVV define. So this PR is essentially just removing REVVEC. From every other users' standpoint it is identical to the devel branch.