Swap byte-order in binary serializer to support big-endian arch (#5813)

* fixed some endian issues

* Use dmlc::ByteSwap() to simplify code

* Fix lint check

* [CI] Add test for s390x

* Download latest CMake on s390x

* Fix a bug in my code

* Save magic number in dmatrix with byteswap on big-endian machine

* Save version in binary with byteswap on big-endian machine

* Load scalar with byteswap in MetaInfo

* Add a debugging message

* Handle arrays correctly when byteswapping

* EOF can also be 255

* Handle magic number in MetaInfo carefully

* Skip Tree.Load test for big-endian, since the test manually builds little-endian binary model

* Handle missing packages in Python tests

* Don't use boto3 in model compatibility tests

* Add s390 Docker file for local testing

* Add model compatibility tests

* Add R compatibility test

* Revert "Add R compatibility test"

This reverts commit c2d2bdcb7dbae133cbb927fcd20f7e83ee2b18a8.

Co-authored-by: Qi Zhang <q.zhang@ibm.com>
Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

tests/python/test_model_compatibility.py

@@ -1,10 +1,12 @@
 import xgboost
 import os
 import generate_models as gm
+import testing as tm
 import json
 import zipfile
 import pytest
 import copy
+import urllib.request
 
 
 def run_model_param_check(config):
@@ -87,6 +89,7 @@ def run_scikit_model_check(name, path):
         assert False
 
 
+@pytest.mark.skipif(**tm.no_sklearn())
 def test_model_compatibility():
     '''Test model compatibility, can only be run on CI as others don't
     have the credentials.
@@ -94,17 +97,9 @@ def test_model_compatibility():
     '''
     path = os.path.dirname(os.path.abspath(__file__))
     path = os.path.join(path, 'models')
-    try:
-        import boto3
-        import botocore
-    except ImportError:
-        pytest.skip(
-            'Skiping compatibility tests as boto3 is not installed.')
-
-    s3_bucket = boto3.resource('s3').Bucket('xgboost-ci-jenkins-artifacts')
-    zip_path = 'xgboost_model_compatibility_test.zip'
-    s3_bucket.download_file(zip_path, zip_path)
 
+    zip_path, _ = urllib.request.urlretrieve('https://xgboost-ci-jenkins-artifacts.s3-us-west-2' +
+                                             '.amazonaws.com/xgboost_model_compatibility_test.zip')
     with zipfile.ZipFile(zip_path, 'r') as z:
         z.extractall(path)