gdb: add inferior_pre_detach observable

Add an observable notified in target_detach just before calling the
detach method on the inferior's target stack.  This allows observer to
do some work on the inferior while it's still ptrace-attached, in the
case of a native Linux inferior.  Specifically, the amd-dbgapi target
will need it in order to call amd_dbgapi_process_detach before the
process gets ptrace-detached.

Change-Id: I28b6065e251012a4c2db8a600fe13ba31671e3c9
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/observable.c

@@ -62,6 +62,7 @@ DEFINE_OBSERVABLE (architecture_changed);
 DEFINE_OBSERVABLE (thread_ptid_changed);
 DEFINE_OBSERVABLE (inferior_added);
 DEFINE_OBSERVABLE (inferior_appeared);
+DEFINE_OBSERVABLE (inferior_pre_detach);
 DEFINE_OBSERVABLE (inferior_exit);
 DEFINE_OBSERVABLE (inferior_removed);
 DEFINE_OBSERVABLE (memory_changed);

gdb/observable.h

@@ -181,6 +181,9 @@ extern observable<struct inferior */* inf */> inferior_added;
    process.  */
 extern observable<struct inferior */* inf */> inferior_appeared;
 
+/* Inferior INF is about to be detached.  */
+extern observable<struct inferior */* inf */> inferior_pre_detach;
+
 /* Either the inferior associated with INF has been detached from
    the process, or the process has exited.  */
 extern observable<struct inferior */* inf */> inferior_exit;

gdb/target.c

@@ -2528,6 +2528,8 @@ target_detach (inferior *inf, int from_tty)
 
   prepare_for_detach ();
 
+  gdb::observers::inferior_pre_detach.notify (inf);
+
   /* Hold a strong reference because detaching may unpush the
      target.  */
   auto proc_target_ref = target_ops_ref::new_reference (inf->process_target ());