[Ada] Simplify dependency on System

[Pierre-Marie de Rodat]

Code cleanup related to loading of compilation units; semantics is
unaffected.

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

gcc/ada/

        * lib-writ.adb (Ensure_System_Dependency): Simplify by reusing a
        constant name.
diff --git a/gcc/ada/lib-writ.adb b/gcc/ada/lib-writ.adb
--- a/gcc/ada/lib-writ.adb
+++ b/gcc/ada/lib-writ.adb
@@ -158,9 +158,7 @@ package body Lib.Writ is
       --  on system as a result of Targparm scanning the system.ads file to
       --  determine the target dependent parameters for the compilation.
 
-      Name_Len := 6;
-      Name_Buffer (1 .. 6) := "system";
-      System_Uname := Name_To_Unit_Name (Name_Enter);
+      System_Uname := Name_To_Unit_Name (Name_System);
       System_Fname := File_Name (System_Source_File_Index);
 
       Units.Increment_Last;