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>

linux-user/riscv/cpu_loop.c

@@ -25,7 +25,7 @@
 
 void cpu_loop(CPURISCVState *env)
 {
-    CPUState *cs = CPU(riscv_env_get_cpu(env));
+    CPUState *cs = env_cpu(env);
     int trapnr, signum, sigcode;
     target_ulong sigaddr;
     target_ulong ret;