cpu: Move singlestep_enabled field from CPU_COMMON to CPUState

Prepares for changing cpu_single_step() argument to CPUState.

Acked-by: Michael Walle <michael@walle.cc> (for lm32)
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/cpus.c b/cpus.c
index 2509eb5..4549b7a 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1186,7 +1186,7 @@
         CPUArchState *env = cpu->env_ptr;
 
         qemu_clock_enable(vm_clock,
-                          (env->singlestep_enabled & SSTEP_NOTIMER) == 0);
+                          (cpu->singlestep_enabled & SSTEP_NOTIMER) == 0);
 
         if (cpu_can_run(cpu)) {
             r = tcg_cpu_exec(env);