Revert "Turn off jemalloc"

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217
test: un-xfail now working run-pass/foreign-struct.rs, close #2334.
test: add testcase, close #4929 which was already fixed.
Add tests for cross-crate condition handling. Close #5446.
Try to fix mac valgrind bot by disabling thread-heavy activities.
compiletest: do not run tests in sub-threads if on valgrind.
rt: make jemalloc header conform to android signature.
auto merge of #8500 : graydon/rust/2013-08-13-self-rollup, r=thestinger

close #8424 r=brson
close #8173 r=brson
close #8209 r=strcat
regenerate configure
jemalloc: use $(AR), not `ar`
auto merge of #8584 : thestinger/rust/jemalloc, r=graydon

This reverts commit 371a316ec93a977f9dcd305b5f532cd394fa6885.

Closes #7217