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/travis/run_test.sh

@@ -88,3 +88,19 @@ if [ ${TASK} == "cmake_test" ]; then
     cd ..
     rm -rf build
 fi
+
+if [ ${TASK} == "s390x_test" ]; then
+    set -e
+
+    # Build and run C++ tests
+    rm -rf build
+    mkdir build && cd build
+    cmake .. -DCMAKE_VERBOSE_MAKEFILE=ON -DGOOGLE_TEST=ON -DUSE_OPENMP=ON -DUSE_DMLC_GTEST=ON -GNinja
+    time ninja -v
+    ./testxgboost
+
+    # Run model compatibility tests
+    cd ..
+    python3 -m pip install --user pytest hypothesis
+    PYTHONPATH=./python-package python3 -m pytest --fulltrace -v -rxXs tests/python/ -k 'test_model'
+fi

tests/travis/setup.sh

@@ -20,6 +20,15 @@ if [ ${TASK} == "cmake_test" ] && [ ${TRAVIS_OS_NAME} == "osx" ]; then
     sudo softwareupdate -i "Command Line Tools (macOS High Sierra version 10.13) for Xcode-9.3"
 fi
 
+if [ ${TASK} == "s390x_test" ] && [ ${TRAVIS_CPU_ARCH} == "s390x" ]; then
+    sudo snap install cmake --channel=3.17/beta --classic
+    export PATH=/snap/bin:${PATH}
+    cmake --version
+    sudo apt-get update
+    sudo apt-get install -y --no-install-recommends tar unzip wget git build-essential ninja-build \
+      time python3 python3-pip python3-numpy python3-scipy python3-sklearn r-base
+fi
+
 if [ ${TASK} == "python_sdist_test" ] && [ ${TRAVIS_OS_NAME} == "linux" ]; then
     wget https://github.com/Kitware/CMake/releases/download/v3.17.1/cmake-3.17.1-Linux-x86_64.sh
     sudo bash cmake-3.17.1-Linux-x86_64.sh --prefix=/usr/local --skip-license