Add Svinval extension.

    These changes add the "Svinval" Standard Extension for Fine-Grained
    Address-Translation Cache Invalidation, Version 1.0 to the sail-riscv model.

    This extension defines five new instructions: SINVAL.VMA, SFENCE.W.INVAL,
    SFENCE.INVAL.IR, HINVAL.VVMA, HINVAL.GVMA.

    HINVAL.VVMA & HINVAL.GVMA are omitted since they build on the
    Hypervisor Extension which is yet to be included in the model.

    SFENCE.W.INVAL & SFENCE.INVAL.IR are treated as nops pending integration
    of the coherency model (rmem) with sail.

    The specification says that SINVAL.VMA behaves just as SFENCE.VMA,
    except there are additional ordering constraints with respect to the new
    SFENCE.W.INVAL & SFENCE.INVAL.IR instructions. Since these are nops, we
    can treat SINVAL.VMA as if it were SFENCE.VMA.