Make jemalloc(3) default to retain:true on 64-bit platforms,
like it already does on Linux and OSX. This results in significantly
fewer calls to mmap(2). This should result in a small reduction
in system CPU time and improved superpage usage.

Reviewed by: markj
Tested by: markj
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23874