gdb, btrace: rename record_btrace_enable_warn()

We use record_btrace_enable_warn() as the new-thread observer callback.
It is not used in other contexts.

Rename it to record_btrace_on_new_thread() to make its role more clear.

gdb/record-btrace.c

@@ -279,10 +279,10 @@ require_btrace (void)
   return &tp->btrace;
 }
 
-/* Enable branch tracing for one thread.  Warn on errors.  */
+/* The new thread observer.  */
 
 static void
-record_btrace_enable_warn (struct thread_info *tp)
+record_btrace_on_new_thread (struct thread_info *tp)
 {
   /* Ignore this thread if its inferior is not recorded by us.  */
   target_ops *rec = tp->inf->target_at (record_stratum);
@@ -306,7 +306,7 @@ record_btrace_auto_enable (void)
 {
   DEBUG ("attach thread observer");
 
-  gdb::observers::new_thread.attach (record_btrace_enable_warn,
+  gdb::observers::new_thread.attach (record_btrace_on_new_thread,
 				     record_btrace_thread_observer_token,
 				     "record-btrace");
 }