[Ada] Add contracts to System.Address_To_Access_Conversions

[Pierre-Marie de Rodat via Gcc-patches]

This patch adds SPARK annotations to subprograms from
System.Address_To_Access_Conversions. To_Pointer is considered to have
no global items, if the returned value has no aliases. To_Address is
forbidden in SPARK because addresses are not handled.

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

gcc/ada/

        * libgnat/s-atacco.ads (To_Pointer): Add Global => null.
        (To_Address): Add SPARK_Mode => Off.
diff --git a/gcc/ada/libgnat/s-atacco.ads b/gcc/ada/libgnat/s-atacco.ads
--- a/gcc/ada/libgnat/s-atacco.ads
+++ b/gcc/ada/libgnat/s-atacco.ads
@@ -54,8 +54,10 @@ package System.Address_To_Access_Conversions is
    --  optimizations that may cause unexpected results based on the assumption
    --  of no strict aliasing.
 
-   function To_Pointer (Value : Address)        return Object_Pointer;
-   function To_Address (Value : Object_Pointer) return Address;
+   function To_Pointer (Value : Address)        return Object_Pointer with
+     Global => null;
+   function To_Address (Value : Object_Pointer) return Address with
+     SPARK_Mode => Off;
 
    pragma Import (Intrinsic, To_Pointer);
    pragma Import (Intrinsic, To_Address);