[Ada][S/390] Fix Memory_Size definition.

-m31 -mzarch uses 64 as word size instead of 32.  This must not affect
the Address type definition which is based on Memory_Size.

gcc/ada/ChangeLog:

2017-04-04  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* system-linux-s390.ads: Use Long_Integer'Size to define
	Memory_Size.

From-SVN: r246678

gcc/ada/ChangeLog

+2017-04-04  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>
+
+	* system-linux-s390.ads: Use Long_Integer'Size to define
+	Memory_Size.
+
 2017-04-04  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* sem_ch3.adb (Build_Derived_Record_Type): Fix long line.

gcc/ada/system-linux-s390.ads

@@ -70,7 +70,7 @@ package System is
 
    Storage_Unit : constant := 8;
    Word_Size    : constant := Standard'Word_Size;
-   Memory_Size  : constant := 2 ** Word_Size;
+   Memory_Size  : constant := 2 ** Long_Integer'Size;
 
    --  Address comparison