target/riscv: Use riscv_csrrw_debug for cpu_dump

Use the official debug read interface to the csrs,
rather than referencing the env slots directly.
Put the list of csrs to dump into a table.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-15-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>