rv64g initial import

[ct-clmsn]

generic risc-v support for g (short hand for the imafdc base extensions)

[insertinterestingnamehere]

Thanks! We're certainly interested in having this. This is a fairly critical section of the code though, so review will take a bit longer.

[ct-clmsn]

Glad to hear there's interest. I'll double back on the smaller issues. The code passes make test on the jh7110 chips I've available. Do you all require floating point registers (and possibly vector registers) to be handled by the assembly that manages context switching?

[insertinterestingnamehere]

Great question! As best I can tell, the correct behavior for the various registers depends on the calling convention for the platform. From the perspective of the end-user, the context swap appears to happen inside an opaque function call. The context swap is supposed to save/restore whatever context is saved/restored by a callee and not cleared away by the caller. It doesn't have to worry about clearing away state that is saved/restored by the caller. I don't know offhand which case applies on RISC-V.