Add a no-op 'android-x86' set of Buildbot steps

This seems to exist before we configure Buildbot to look for it.
Actual compilation and testing steps will be added once Buildbot
is configured to run them.

CC https://github.com/servo/servo/issues/21124,
https://github.com/servo/servo/issues/21116

etc/ci/buildbot_steps.yml

@@ -159,6 +159,9 @@ android:
   - bash ./etc/ci/lockfile_changed.sh
   - python ./etc/ci/check_dynamic_symbols.py
 
+android-x86:
+  - echo FIXME To be filled in once Buildbot is configured
+
 android-nightly:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force