Revert "Turn off jemalloc"

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217
rt: make jemalloc header conform to android signature.
regenerate configure
jemalloc: use $(AR), not `ar`
auto merge of #8584 : thestinger/rust/jemalloc, r=graydon

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217