Use external jemalloc

[lucab]

It looks like we are currently bundling a pretty standard jemalloc 3.4.0.
It would be good to have a configure switch to avoid re-building libjemalloc in rust and just use the system-wide.

[alexcrichton]

jemalloc was removed in #9933, so I'm going to close this. If we decide to re-integrate jemalloc, this has the possibility of becoming an issue, but thankfully github keeps archives of everything!