Build libuv/jemalloc only once (not per stage)

Reorganize the makefiles to stop building these once per stage because there's
no need to do this at all.

Closes #7002
auto merge of #9772 : alexcrichton/rust/buildsystem, r=thestinger

Reorganize the makefiles to stop building these once per stage because there's
no need to do this at all.

Closes #7002