Fix various small build issues, Java API naming (#7776)

Summary:
* Compatibility with older GCC.
* Compatibility with older jemalloc libraries.
* Remove Docker warning when building i686 binaries.
* Fix case inconsistency in Java API naming (potential update to HISTORY.md deferred)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7776

Reviewed By: akankshamahajan15

Differential Revision: D25607235

Pulled By: pdillinger

fbshipit-source-id: 7ab0fb7fa7a34e97ed0bec991f5081acb095777d