MB-12067: Use jemalloc as the preferred allocator on Linux/OS X

To facilitate using ep-engine's Defragmenter task, change the order of
prescidence of memory allocators so that jemalloc is preferred above
TCMalloc on Linux & OS X. Windows will continue to prefer TCMalloc.

Change-Id: Icaf678f04b28e5b1f54a60b1f711a226ebb95ef0
Reviewed-on: http://review.couchbase.org/43363
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Mark Woosey <mark.woosey@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>