gdbstub: Set gdb_set_stop_cpu() argument to CPUState

Use CPUState::env_ptr for now.

Prepares for changing cpu_handle_guest_debug() argument to CPUState.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/cpus.c b/cpus.c
index 12f6f32..54acced 100644
--- a/cpus.c
+++ b/cpus.c
@@ -463,7 +463,7 @@
 {
     CPUState *cpu = ENV_GET_CPU(env);
 
-    gdb_set_stop_cpu(env);
+    gdb_set_stop_cpu(cpu);
     qemu_system_debug_request();
     cpu->stopped = true;
 }