Auto merge of #21779 - servo:jdm-patch-30, r=Manishearth

Create build steps for android-mac

This will ensure that cross-compiling from mac->android does not regress in the future.

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etc/ci/buildbot_steps.yml

@@ -185,6 +185,16 @@ android:
   - python ./etc/ci/check_dynamic_symbols.py
   - ./etc/ci/clean_build_artifacts.sh
 
+android-mac:
+ commands:
+  - ./mach clean-nightlies --keep 3 --force
+  - ./mach clean-cargo-cache --keep 3 --force
+  - ./etc/ci/bootstrap-android-and-accept-licences.sh
+  - env --unset ANDROID_NDK --unset ANDROID_SDK ./mach build --android --dev
+  - env --unset ANDROID_NDK --unset ANDROID_SDK ./mach package --android --dev
+  - bash ./etc/ci/lockfile_changed.sh
+  - ./etc/ci/clean_build_artifacts.sh
+
 android-x86:
  env:
    HOST_CC: gcc-5