Move singlestep_enabled to CPUState.

The previous patch forgot to move this field from CPU_COMMON to
CPUState, so do it here.

Change-Id: I71ed4605c939eb6c23d3e70b08ea89e59842cdd0
diff --git a/gdbstub.c b/gdbstub.c
index 92c4153..28cfe1c 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1650,7 +1650,7 @@
             addr = strtoull(p, (char **)&p, 16);
             gdb_set_cpu_pc(s, addr);
         }
-        cpu_single_step(s->c_cpu, sstep_flags);
+        cpu_single_step(ENV_GET_CPU(s->c_cpu), sstep_flags);
         gdb_continue(s);
 	return RS_IDLE;
     case 'F':
@@ -1971,7 +1971,7 @@
         return;
 
     /* disable single step if it was enable */
-    cpu_single_step(env, 0);
+    cpu_single_step(ENV_GET_CPU(env), 0);
 
     if (reason == EXCP_DEBUG) {
         if (env->watchpoint_hit) {