target/tilegx: Use env_cpu

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

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

linux-user/tilegx/cpu_loop.c

@@ -206,7 +206,7 @@ static void do_fetch(CPUTLGState *env, int trapnr, bool quad)
 
 void cpu_loop(CPUTLGState *env)
 {
-    CPUState *cs = CPU(tilegx_env_get_cpu(env));
+    CPUState *cs = env_cpu(env);
     int trapnr;
 
     while (1) {