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_early_stopping.py

@@ -82,6 +82,7 @@ class TestEarlyStopping(unittest.TestCase):
         self.assert_metrics_length(cv, 1)
 
     @pytest.mark.skipif(**tm.no_sklearn())
+    @pytest.mark.skipif(**tm.no_pandas())
     def test_cv_early_stopping_with_multiple_eval_sets_and_metrics(self):
         from sklearn.datasets import load_breast_cancer