[Ada] Refine change for bit-packed slices

We use Long_Long_Integer'Size / 2 instead of 4 * 8 to support
specifying a target configuration file where the largest integer is
32 bits instead of 64.

2019-09-17  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* libgnat/s-bitfie.ads (Val_Bits, Val_Bytes): Define from
	Long_Long_Integer'Size.

From-SVN: r275773

gcc/ada/ChangeLog

+2019-09-17  Arnaud Charlet  <charlet@adacore.com>
+
+	* libgnat/s-bitfie.ads (Val_Bits, Val_Bytes): Define from
+	Long_Long_Integer'Size.
+
 2019-09-17  Javier Miranda  <miranda@adacore.com>
 
 	* exp_ch6.ads (Needs_BIP_Task_Actuals): New subprogram.

gcc/ada/libgnat/s-bitfie.ads

@@ -36,9 +36,13 @@ package System.Bitfields is
    --  Instances of the generic package in System.Bitfield_Utils. So far
    --  we have just one, which defaults to the natural endianness of the
    --  machine. We might someday want to support Scalar_Storage_Order.
+   --  Note: we use Long_Long_Integer'Size / 2 instead of 32 to support
+   --  specifying a target configuration file where the largest integer is
+   --  32 bits instead of 64.
+
+   Val_Bits  : constant := Long_Long_Integer'Size / 2;
+   Val_Bytes : constant := Val_Bits / System.Storage_Unit;
 
-   Val_Bytes : constant := 4;
-   Val_Bits : constant := Val_Bytes * System.Storage_Unit;
    type Val_2 is mod 2**(Val_Bits * 2) with Alignment => Val_Bytes;
    pragma Provide_Shift_Operators (Val_2);
    type Val is mod 2**Val_Bits with Alignment => Val_Bytes;