Stop building jemalloc once per stage

[bstrie]

18:57 < bstrie> cmr: it looks like jemalloc gets built at every stage, rather 
                than once ever. surely that's not intentional?
18:57 < strcat> bstrie: the runtime is built at every stage now
18:57 < strcat> it wasn't when I made that
18:58 < bstrie> strcat: surely there's no reason we *have* to build jemalloc at 
                every stage, right?
18:58 < strcat> there isn't really a reason but it means making another 
                top-level directory for it
18:59 < strcat> where do we put libuv?
18:59 < cmr> Would an extern or thirdparty dir make sense?
18:59 < bstrie> I think handling it the same way as libuv would be ideal
18:59 < bstrie> for the sake of consistency

[alexcrichton]

Blocked on #7217

[huonw]

Triage: jemalloc is renabled, so this is is again relevant, and the logs indicate that we're building (and configuring!) jemalloc anew at each stage.