Pick f80c97e477d1b3fe7778c65d9439d673738b4131 from upstream:
Rework the way jemalloc uses mmap(2) on FreeBSD. This makes it directly use MAP_EXCL and MAP_ALIGNED() instead of weird workarounds involving mapping at random places and then unmapping parts of them.
Discussed with: jasone
MFC after: 2 weeks
Sponsored by: DARPA, AFRL