gdbstub: Simplify find_cpu()

Use qemu_get_cpu() and CPUState::env_ptr.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

gdbstub.c

@@ -2071,17 +2071,13 @@ static void gdb_set_cpu_pc(GDBState *s, target_ulong pc)
 
 static CPUArchState *find_cpu(uint32_t thread_id)
 {
-    CPUArchState *env;
     CPUState *cpu;
 
-    for (env = first_cpu; env != NULL; env = env->next_cpu) {
-        cpu = ENV_GET_CPU(env);
-        if (cpu_index(cpu) == thread_id) {
-            return env;
-        }
+    cpu = qemu_get_cpu(thread_id);
+    if (cpu == NULL) {
+        return NULL;
     }
-
-    return NULL;
+    return cpu->env_ptr;
 }
 
 static int gdb_handle_packet(GDBState *s, const char *line_buf)