rtl: StringOfChar: use repeat if available

packages/rtl/system.pas

@@ -603,6 +603,9 @@ function StringOfChar(c: Char; l: NativeInt): String;
 var
   i: Integer;
 begin
+  asm
+    if (c.repeat) return c.repeat(l);
+  end;
   Result:='';
   for i:=1 to l do Result:=Result+c;
 end;