Summary:
`USE_LTO=1` in `make` commands now enables LTO. The archiver (`ar`) needed
to change in this PR to use a wrapper that enables the LTO plugin.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7181

Test Plan:
build a few ways
```
$ make clean && USE_LTO=1 make -j48 db_bench
$ make clean && USE_CLANG=1 USE_LTO=1 make -j48 db_bench
$ make clean && ROCKSDB_NO_FBCODE=1 USE_LTO=1 make -j48 db_bench
```

Reviewed By: cheng-chang

Differential Revision: D22784994

Pulled By: ajkr

fbshipit-source-id: 9c45333bd49bf4615aa04c85b7c6fd3925421152