Reenable jemalloc

[brson]

I turned it off because it causes canary kills in compile-fail tests.

[alexcrichton]

Nominating production-ready.

[bstrie]

I think that @thestinger was concerned that, as long as split stacks are supported (not just enabled-by-default, just supported at all), jemalloc cannot be used with Rust. Is this correct?

[brson]

@bstrie I can't imagine why that would be true.

I think we should turn on jemalloc now while split stacks are disabled.