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>

target/tilegx/helper.c

@@ -28,7 +28,7 @@
 
 void helper_exception(CPUTLGState *env, uint32_t excp)
 {
-    CPUState *cs = CPU(tilegx_env_get_cpu(env));
+    CPUState *cs = env_cpu(env);
 
     cs->exception_index = excp;
     cpu_loop_exit(cs);