Change target_ops::async to accept bool

This changes the parameter of target_ops::async from int to bool.
Regression tested on x86-64 Fedora 34.

gdb/fbsd-nat.h

@@ -68,7 +68,7 @@ public:
 
   bool can_async_p () override;
 
-  void async (int) override;
+  void async (bool) override;
 
   thread_control_capabilities get_thread_control_capabilities () override
   { return tc_schedlock; }