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_demos.py

@@ -109,6 +109,8 @@ def test_evals_result_demo():
     subprocess.check_call(cmd)
 
 
+@pytest.mark.skipif(**tm.no_sklearn())
+@pytest.mark.skipif(**tm.no_pandas())
 def test_aft_demo():
     script = os.path.join(DEMO_DIR, 'aft_survival', 'aft_survival_demo.py')
     cmd = ['python', script]