problem debugging ravenscar programs if runtime is stripped

Trying to debug a program using a stripped version of the ravenscar
runtime, we can get the following error:

    (gdb) cont
    Continuing.
    Cannot find Ada_Task_Control_Block type. Aborting

This is because the ravenscar-thread layer makes the assumption that
the runtime is built the way we expect it, meaning that the Ada tasking
units we rely on for Ada tasking debugging, are built with debugging
information, and that this debug information has not been stripped from
the runtime.

When this assumption is not true, resuming such a program can trigger
the error above, which then leads GDB a little confused. For instance,
we can see things like:

     (gdb) bt
     Target is executing.

This patch fixes the issue by disabling the ravenscar thread layer
if we detect that the runtime is missing some of the debugging info
we need in order to support Ada task debugging. This is the best
we can do, as the ravenscar-thread layer actually depends on the
ada-tasks layer to implement thread debugging.

gdb/ChangeLog:

        * ada-lang.h (ada_get_tcb_types_info): Add declaration.
        * ada-tasks.c (ada_get_tcb_types_info): Renames get_tcb_types_info.
        Make non-static.  Change return type to char *.  Adjust code
        accordingly.  Rewrite the function's documentation.
        (read_atcb): Adjust call to get_tcb_types_info accordingly.
        * ravenscar-thread.c (ravenscar_inferior_created): Check that
        we have enough debugging information in the runtime to support
        Ada task debugging before we enable the ravenscar-thread layer.

gdb/ada-lang.h

@@ -409,6 +409,8 @@ typedef void (ada_task_list_iterator_ftype) (struct ada_task_info *task);
 extern void iterate_over_live_ada_tasks
   (ada_task_list_iterator_ftype *iterator);
 
+extern const char *ada_get_tcb_types_info (void);
+
 extern int ada_build_task_list (void);
 
 extern void print_ada_task_info (struct ui_out *uiout,