Summary:
Contrary to other systems, jemalloc is part of the FreeBSD's libc.
As such APIs are directly exposed. Enabling super pages
as well, supporting same page size.
Pull Request resolved: https://github.com/facebook/folly/pull/1294

Reviewed By: interwq

Differential Revision: D19249354

Pulled By: yfeldblum

fbshipit-source-id: 845f594b28f30b0360d4573d8a7354f9790bc8b7