[libc++] Re-enable backdeployment testing on arm64 macOS

libcxx/utils/ci/buildkite-pipeline.yml

@@ -121,6 +121,14 @@ steps:
       arch: x86_64 # TODO: Remove this once we are able to run back-deployment on arm64 again, since this isn't x86_64 specific
     <<: *common
 
+  - label: "Apple back-deployment macosx11.0 arm64"
+    command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-11.0"
+    agents:
+      queue: libcxx-builders
+      os: macos
+      arch: arm64
+    <<: *common
+
 - group: ARM
   steps:
   - label: AArch64
@@ -230,20 +238,3 @@ steps:
       queue: libcxx-builders
       os: android
     <<: *common
-
-
-    # TODO: Re-enable this once we've figured out how to run back-deployment testing on arm64 on recent OSes
-    # - label: "Apple back-deployment macosx11.0 arm64"
-    #   command: "libcxx/utils/ci/run-buildbot apple-system-backdeployment-11.0"
-    #   artifact_paths:
-    #     - "**/test-results.xml"
-    #     - "**/*.abilist"
-    #   agents:
-    #     queue: "libcxx-builders"
-    #     os: "macos"
-    #     arch: "arm64"
-    #   retry:
-    #     automatic:
-    #       - exit_status: -1  # Agent was lost
-    #         limit: 2
-    #   timeout_in_minutes: 120