[CI] Set up CI for Mac M1 (#9699)

tests/buildkite/pipeline-mac-m1.yml

@@ -0,0 +1,8 @@
+steps:
+  - block: ":rocket: Run this test job"
+    if: build.pull_request.id != null || build.branch =~ /^dependabot\//
+  - label: ":macos: Build and Test XGBoost for MacOS M1 with Clang 11"
+    command: "tests/buildkite/test-macos-m1-clang11.sh"
+    key: mac-m1-appleclang11
+    agents:
+      queue: mac-mini-m1

tests/buildkite/test-macos-m1-clang11.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source tests/buildkite/conftest.sh
+
+# Display system info
+echo "--- Display system information"
+set -x
+system_profiler SPSoftwareDataType
+sysctl -n machdep.cpu.brand_string
+uname -m
+set +x
+
+# Create new Conda env
+echo "--- Set up Conda env"
+. $HOME/mambaforge/etc/profile.d/conda.sh
+. $HOME/mambaforge/etc/profile.d/mamba.sh
+conda_env=xgboost_dev_$(uuidgen | tr '[:upper:]' '[:lower:]' | tr -d '-')
+mamba create -y -n ${conda_env} python=3.8
+conda activate ${conda_env}
+mamba env update -n ${conda_env} --file tests/ci_build/conda_env/macos_cpu_test.yml
+
+# Ensure that XGBoost can be built with Clang 11
+echo "--- Build and Test XGBoost with MacOS M1, Clang 11"
+set -x
+LLVM11_PATH=$(brew --prefix llvm\@11)
+mkdir build
+pushd build
+cmake .. -GNinja -DCMAKE_C_COMPILER=${LLVM11_PATH}/bin/clang \
+  -DCMAKE_CXX_COMPILER=${LLVM11_PATH}/bin/clang++ -DGOOGLE_TEST=ON \
+  -DUSE_DMLC_GTEST=ON
+ninja -v