Fix Huffman decoding on big-endian systems

On PPC systems, HTTP2 & HPACK testcases were failing because
AWS.HTTP2.HPACK.Huffman.Decode was raising Protocol_Errors over inconsistent
padding.

The encoded bytestrings were correct; things were diverging in
AWS.HTTP2.HPACK.Decode, where bytes were misinterpreted when read as
RFC_Byte, due to that record's representation clause being target-dependent.

For example, if we break in that function right as it processes the first
byte of an encoded string, and that first byte is decimal integer 130…

  256            Byte := Get_Byte;
  (gdb) next
  258            if Bit.B0 = 1 then
  (gdb) p/t byte
  $8 = 10000010

In RFC_Byte's "Bₙ at 0 range i .. j" component clauses, by default, i and j
are interpreted differently depending on which bit ordering the target uses.
Thus on a little-endian platform like x86_64-linux:

  (gdb) p/t bit
  $9 = (
      b7 => 0,
      b6 => 1,
      b5 => 0,
      b4 => 0,
      b3 => 0,
      b2 => 0,
      b1 => 0,
      b0 => 1,
      b23 => 10,
      b22 => 0,
      b21 => 0,
      b20 => 10,
      b30 => 100,
      b41 => 10,
      b40 => 1000
    )

Whereas on a big-endian platform such as ppc64-linux:

  (gdb) p/t bit
  $9 = (
      b7 => 1,
      b6 => 0,
      b5 => 0,
      b4 => 0,
      b3 => 0,
      b2 => 0,
      b1 => 1,
      b0 => 0,
      b23 => 10,
      b22 => 0,
      b21 => 0,
      b20 => 10,
      b30 => 10,
      b41 => 1000,
      b40 => 10
    )

Fix this by picking an explicit Bit_Order for RFC_Byte, so that in those
range clauses, i and j are interpreted consistently across all platforms
(i.e. pick Low_Order_First so that "0" refers to the MSb, "7" to the LSb).

Also fix AWS.HTTP2.HPACK.Huffman.Decode, which presents a similar problem: a
Stream_Element is aliased as an array of bits, but the indexing order of that
array will depend on endianness.  Use a similar solution to

  2023-03-21 "Fix creation of Huffman decoding tree for big-endian
  platforms" (7eca5a10c)

That is, remove the aliasing array type, and rely on platform-independent
shifts.

TN: eng/toolchain/aws#4

regtests/0341_hpack/test.opt

@@ -1,2 +0,0 @@
-ppc-linux XFAIL Server crash; see eng/toolchain/aws#4.
-ppc64-linux XFAIL Server crash; see eng/toolchain/aws#4.

regtests/0344_http2_hello/test.opt

@@ -1,2 +0,0 @@
-ppc-linux XFAIL Server crash; see eng/toolchain/aws#4.
-ppc64-linux XFAIL Server crash; see eng/toolchain/aws#4.

regtests/0346_http2_post_attachments/test.opt

@@ -1,2 +0,0 @@
-ppc-linux XFAIL Server crash; see eng/toolchain/aws#4.
-ppc64-linux XFAIL Server crash; see eng/toolchain/aws#4.

regtests/0347_http2_post_connection/test.opt

@@ -1,2 +0,0 @@
-ppc-linux XFAIL Server crash; see eng/toolchain/aws#4.
-ppc64-linux XFAIL Server crash; see eng/toolchain/aws#4.

regtests/0348_http2_client_status/test.opt

@@ -1,2 +0,0 @@
-ppc-linux XFAIL Server crash; see eng/toolchain/aws#4.
-ppc64-linux XFAIL Server crash; see eng/toolchain/aws#4.

src/http2/aws-http2-hpack-huffman.adb

@@ -380,7 +380,6 @@ package body AWS.HTTP2.HPACK.Huffman is
 
       EOS    : constant Unsigned_32 := 16#fffffffa#;
 
-      type Byte_Bits is array (0 .. 7) of Bit with Pack;
       Iter    : Node_Access := Root;
       Padding : Natural := 0;
       Pad_0   : Boolean := False;
@@ -390,7 +389,6 @@ package body AWS.HTTP2.HPACK.Huffman is
       for K in Str'Range loop
          declare
             E    : constant Stream_Element := Str (K);
-            Bits : Byte_Bits with Size => 8, Address => E'Address;
             C    : Character;
          begin
             --  Keep last four bytes to check for EOS
@@ -406,7 +404,8 @@ package body AWS.HTTP2.HPACK.Huffman is
 
             for B in reverse 0 .. 7 loop
                declare
-                  Bit : constant Huffman.Bit := Bits (B);
+                  Bit : constant Huffman.Bit :=
+                          Huffman.Bit (Shift_Right (Unsigned_8 (E), B) and 1);
                begin
                   if Decode_Bit (Iter, Bit, C) then
                      I := I + 1;

src/http2/aws-http2-hpack.adb

@@ -28,6 +28,7 @@
 ------------------------------------------------------------------------------
 
 with Interfaces;
+with System;
 
 with AWS.HTTP2.Connection;
 with AWS.HTTP2.HPACK.Huffman;
@@ -93,6 +94,11 @@ package body AWS.HTTP2.HPACK is
       B41 at 0 range 0 .. 3;
    end record;
 
+   --  Pick an explicit bit indexing order for this record, otherwise
+   --  the position & ranges in the component clauses will be
+   --  interpreted differently depending on endianness.
+   for RFC_Byte'Bit_Order use System.Low_Order_First;
+
    B_II          : constant Bit2 := 2#01#;
    --  Incremental Indexing