target/riscv: move RISCVCPUConfig fields to a header file

To support merging a subclass's RISCVCPUDef into the superclass, a list
of all the CPU features is needed.  Put them into a header file that
can be included multiple times, expanding the macros BOOL_FIELD and
TYPE_FIELD to different operations.

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