makefile: jemalloc is re-built on every `make check` / `make check-stage1` etc

[pnkfelix]

jemalloc is being re-built on every make check / make check-stage1 etc.

We manage to avoid doing such rebuilds for libuv and llvm. We should figure out what makefile hacking is necessary to give jemalloc the same treatment.

[pnkfelix]

part of #8058

[alexcrichton]

I think this has since been fixed.