Revert "Turn off jemalloc"

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217
rt: make jemalloc header conform to android signature.
auto merge of #8628 : graydon/rust/2013-08-19-jemalloc-enable, r=thestinger

Taking over from #8584.