Revert "Turn off jemalloc"

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217
auto merge of #8584 : thestinger/rust/jemalloc, r=brson

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217