target/riscv: merge riscv_cpu_class_init with the class_base function

Since all TYPE_RISCV_CPU subclasses support a class_data of type
RISCVCPUDef, process it even before calling the .class_init function
for the subclasses.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>