[Ada] Unsynchronized access to a Boolean in tasking state

[Pierre-Marie de Rodat]

The Terminated flag for a task was being queried without first aquiring
the task lock. It is not clear that this unsychronized access has ever
caused a problem in practice, but the thread-sanitizer tool flags it.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * libgnarl/s-tassta.adb (Free_Task): Acquire the Task_Lock
        before, rather than after, querying the task's Terminated flag.
        Add a corresponding Task_Unlock call.
diff --git a/gcc/ada/libgnarl/s-tassta.adb b/gcc/ada/libgnarl/s-tassta.adb
--- a/gcc/ada/libgnarl/s-tassta.adb
+++ b/gcc/ada/libgnarl/s-tassta.adb
@@ -910,12 +910,12 @@ package body System.Tasking.Stages is
       Self_Id : constant Task_Id := Self;
 
    begin
+      Initialization.Task_Lock (Self_Id);
+
       if T.Common.State = Terminated then
 
          --  It is not safe to call Abort_Defer or Write_Lock at this stage
 
-         Initialization.Task_Lock (Self_Id);
-
          Lock_RTS;
          Initialization.Finalize_Attributes (T);
          Initialization.Remove_From_All_Tasks_List (T);
@@ -930,6 +930,7 @@ package body System.Tasking.Stages is
          --  upon termination.
 
          T.Free_On_Termination := True;
+         Initialization.Task_Unlock (Self_Id);
       end if;
    end Free_Task;