target/alpha: Use env_cpu, env_archcpu

Cleanup in the boilerplate that each target must define.
Replace alpha_env_get_cpu with env_archcpu.  The combination
CPU(alpha_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/alpha/cpu_loop.c

@@ -23,7 +23,7 @@
 
 void cpu_loop(CPUAlphaState *env)
 {
-    CPUState *cs = CPU(alpha_env_get_cpu(env));
+    CPUState *cs = env_cpu(env);
     int trapnr;
     target_siginfo_t info;
     abi_long sysret;