[Ada] Replace low-level condition with a high-level call

[Pierre-Marie de Rodat]

Code cleanup; semantics is unaffected.

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

gcc/ada/

        * lib-writ.adb (Ensure_System_Dependency): Simplify condition.
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
@@ -147,7 +147,7 @@ package body Lib.Writ is
    begin
       --  Nothing to do if we already compiled System
 
-      if Unit_Names.Get (System_Uname) /= No_Unit then
+      if Is_Loaded (System_Uname) then
          return;
       end if;