Add ubuntu and macOS arm64 tests

In response to #374
(For my future reference: The list of bazel ci supported platforms is in PLATFORMS, here: https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/bazelci.py)

.bazelci/presubmit.yml

@@ -12,12 +12,24 @@ tasks:
     - "//..."
     test_targets:
     - "//..."
+  ubuntu2004_arm64:
+    working_directory: test
+    build_targets:
+    - "//..."
+    test_targets:
+    - "//..."
   macos:
     working_directory: test
     build_targets:
     - "//..."
     test_targets:
     - "//..."
+  macos_arm64:
+    working_directory: test
+    build_targets:
+    - "//..."
+    test_targets:
+    - "//..."
   windows:
     working_directory: test
     environment: