cpu: Make first_cpu and next_cpu CPUState

Move next_cpu from CPU_COMMON to CPUState.
Move first_cpu variable to qom/cpu.h.

gdbstub needs to use CPUState::env_ptr for now.
cpu_copy() no longer needs to save and restore cpu_next.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
[AF: Rebased, simplified cpu_copy()]
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/linux-user/main.c b/linux-user/main.c
index af82db8..564bed6 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -120,8 +120,8 @@
     if (child) {
         /* Child processes created by fork() only have a single thread.
            Discard information about the parent threads.  */
-        first_cpu = thread_env;
-        thread_env->next_cpu = NULL;
+        first_cpu = ENV_GET_CPU(thread_env);
+        first_cpu->next_cpu = NULL;
         pending_cpus = 0;
         pthread_mutex_init(&exclusive_lock, NULL);
         pthread_mutex_init(&cpu_list_mutex, NULL);
@@ -148,7 +148,6 @@
    Must only be called from outside cpu_arm_exec.   */
 static inline void start_exclusive(void)
 {
-    CPUArchState *other;
     CPUState *other_cpu;
 
     pthread_mutex_lock(&exclusive_lock);
@@ -156,8 +155,7 @@
 
     pending_cpus = 1;
     /* Make all other cpus stop executing.  */
-    for (other = first_cpu; other; other = other->next_cpu) {
-        other_cpu = ENV_GET_CPU(other);
+    for (other_cpu = first_cpu; other_cpu; other_cpu = other_cpu->next_cpu) {
         if (other_cpu->running) {
             pending_cpus++;
             cpu_exit(other_cpu);