[Ada] Import Adafinal as convention Ada

[Pierre-Marie de Rodat]

This is in general harmless but can cause inconsistencies on some
targets.

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

2020-06-04  Arnaud Charlet  <char...@adacore.com>

gcc/ada/

        * bindgen.adb (Gen_Adafinal): Adafinal is convention Ada, not C.
--- gcc/ada/bindgen.adb
+++ gcc/ada/bindgen.adb
@@ -457,7 +457,7 @@ package body Bindgen is
 
       if not Bind_For_Library and not CodePeer_Mode then
          WBI ("      procedure s_stalib_adafinal;");
-         Set_String ("      pragma Import (C, s_stalib_adafinal, ");
+         Set_String ("      pragma Import (Ada, s_stalib_adafinal, ");
          Set_String ("""system__standard_library__adafinal"");");
          Write_Statement_Buffer;
       end if;