[Ada] Add one more leading underscore to couple of exported symbols

[Pierre-Marie de Rodat via Gcc-patches]

For the sake of consistency with other runtime units.

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

gcc/ada/

        * libgnat/s-stchop.ads: Use a double underscore prefix for symbols.
diff --git a/gcc/ada/libgnat/s-stchop.ads b/gcc/ada/libgnat/s-stchop.ads
--- a/gcc/ada/libgnat/s-stchop.ads
+++ b/gcc/ada/libgnat/s-stchop.ads
@@ -72,7 +72,7 @@ package System.Stack_Checking.Operations is
 private
    Cache : aliased Stack_Access := Null_Stack;
 
-   pragma Export (C, Cache, "_gnat_stack_cache");
-   pragma Export (C, Stack_Check, "_gnat_stack_check");
+   pragma Export (C, Cache, "__gnat_stack_cache");
+   pragma Export (C, Stack_Check, "__gnat_stack_check");
 
 end System.Stack_Checking.Operations;