Handling of V extension is incorrect

[Timmmm]

We currently treat V as meaning "vector is supported", as you might logically expect.

In fact it doesn't mean that - it means vector is supported and vlen >= 128 and elen == 64 (and F & D support).

This affects the ISA string, and misa.v.

I think the simplest fix, congruent with our planned handling of Zvl128 etc. is:

1. Change the "vector is supported" knob in the config from extensions.V.supported to something like base.vector_supported.
2. Change

function clause hartSupports(Ext_V) = config extensions.V.supported

to something like:

function vectorSupported() -> bool = config base.vector_supported

function clause hartSupports(Ext_V) = vectorSupported() & vlen >= 128 & elen == 64

Similarly for currentlyEnabled(Ext_V).

Open to other suggestions though.

[nadime15]

For those who are curious, we have started some of the discussion here

[jordancarlin]

function vectorSupported() -> bool = config base.vector_supported

function clause hartSupports(Ext_V) = vectorSupported() & vlen >= 128 & elen == 64

Unfortunately this is not quite accurate because V enables a few extra extensions that you do not technically need to support even if you have vlen >= 128 & elen == 64. We still need a V.supported option in the config file separate from base.vector_supported.