disas/riscv: Move types/constants to new header file

disas/riscv: Move types/constants to new header file

In order to enable vendor disassembler support, we need to
move types and constants into a header file so that other
compilation units can use them as well.

This patch does not introduce any functional changes.

Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-Id: <20230612111034.3955227-4-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>