Add macos_arm64 platform (#1123)

1. Add macos_arm64 as a new platform, but underlying we still use x86_64
   machines to do cross-compiling. We'll switch to actual Apple Silicon
   machines when they are available.

2. Publish cross-compiled arm64 Bazel binary. Note that the binary is
   not yet well tested on Apple Silicon.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 7efafc3..9c758d2 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -517,6 +517,16 @@
         "queue": "macos",
         "python": "python3",
     },
+    "macos_arm64": {
+        "name": "macOS (arm64), OpenJDK 8",
+        "emoji-name": ":darwin: (arm64) (OpenJDK 8)",
+        "downstream-root": "/Users/buildkite/builds/${BUILDKITE_AGENT_NAME}/${BUILDKITE_ORGANIZATION_SLUG}-downstream-projects",
+        "publish_binary": [], # Change to ["macos_arm64"] when bazel_publish_binaries.yml is updated.
+        # TODO(pcloudy): Switch to macos_arm64 queue when Apple Silicon machines are available,
+        # current we just use x86_64 machines to do cross compile.
+        "queue": "macos",
+        "python": "python3",
+    },
     "windows": {
         "name": "Windows, OpenJDK 8",
         "emoji-name": ":windows: (OpenJDK 8)",