target-alpha: Initialize env->cpu_model_str

Save the cpu_model_str so that we have a non-null value when
creating a new cpu during clone.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

target-alpha/translate.c

@@ -3543,6 +3543,7 @@ CPUAlphaState * cpu_alpha_init (const char *cpu_model)
     }
     env->implver = implver;
     env->amask = amask;
+    env->cpu_model_str = cpu_model;
 
     qemu_init_vcpu(env);
     return env;