cpu: Use QTAILQ for CPU list

Introduce CPU_FOREACH(), CPU_FOREACH_SAFE() and CPU_NEXT() shorthand
macros.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 7ce2eab..72d9270 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -2668,7 +2668,7 @@
 
     /* read and fill status of all threads */
     cpu_list_lock();
-    for (cpu = first_cpu; cpu != NULL; cpu = cpu->next_cpu) {
+    CPU_FOREACH(cpu) {
         if (cpu == thread_cpu) {
             continue;
         }