[gcc r16-6605] ada: Fix fallout of recent change related to task activation

[Marc Poulhies]

https://gcc.gnu.org/g:6ee1f585e5a046c7137b44696a2f0192c2f4d553

commit r16-6605-g6ee1f585e5a046c7137b44696a2f0192c2f4d553
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Wed Nov 26 16:20:39 2025 +0100

    ada: Fix fallout of recent change related to task activation
    
    The Move_Activation_Chain routine is not available in all runtime libraries.
    
    gcc/ada/ChangeLog:
    
            * exp_ch6.adb (Expand_N_Extended_Return_Statement): Do not call
            Move_Activation_Chain if it is not available in the runtime library.

Diff:
---
 gcc/ada/exp_ch6.adb | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/gcc/ada/exp_ch6.adb b/gcc/ada/exp_ch6.adb
index 48627649a309..981cb2c86840 100644
--- a/gcc/ada/exp_ch6.adb
+++ b/gcc/ada/exp_ch6.adb
@@ -5920,6 +5920,12 @@ package body Exp_Ch6 is
 
       function Move_Activation_Chain (Func_Id : Entity_Id) return Node_Id is
       begin
+         --  Move_Activation_Chain is not universally available
+
+         if not RTE_Available (RE_Move_Activation_Chain) then
+            return Make_Null_Statement (Loc);
+         end if;
+
          return
            Make_Procedure_Call_Statement (Loc,
              Name                   =>