target/riscv: introduce RISCVCPUDef

target/riscv: introduce RISCVCPUDef

Start putting all the CPU definitions in a struct.  Later this will replace
instance_init functions with declarative code, for now just remove the
ugly cast of class_data.

Reviewed-by: Alistair Francis <alistair23@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>