Summary:
Allow arcanist_util to work with both new and old arc versions.
The diff is based on Adam Retter's pull request
https://github.com/facebook/rocksdb/pull/1168
Many thanks to Adam to initiate this work
Test Plan: run arc lint and arc diff using different arc versions
Reviewers: sdong, IslamAbdelRahman, kradhakrishnan, andrewkr, adamretter, igor
Reviewed By: adamretter
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D59697
