Summary:
Loose ends relate to mmap on 32-bit systems. (Testing is more
complicated when the feature was completely disabled on 32-bit.)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9386

Test Plan: CI

Reviewed By: ajkr

Differential Revision: D33590715

Pulled By: pdillinger

fbshipit-source-id: f2637036a538a552200adee65b6765fce8cae27b