target/riscv: Use env_cpu, env_archcpu

target/riscv: Use env_cpu, env_archcpu

Cleanup in the boilerplate that each target must define.
Replace riscv_env_get_cpu with env_archcpu.  The combination
CPU(riscv_env_get_cpu) should have used ENV_GET_CPU to begin;
use env_cpu now.

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