Summary:
`-O3` is already adopted widely, so we should make it easier to configure
for development/open source. This PR adds an `OPTIMIZE_LEVEL` variable
that users can set to override the `-O` flag chosen in the Makefile.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7202

Test Plan: built a few different ways and verified correct value is passed for `-O` flag

Reviewed By: pdillinger

Differential Revision: D22845291

Pulled By: ajkr

fbshipit-source-id: 84471362e7d627dd606b25bf5f6a3d796817fa1c