Build broken with --disable-jemalloc

[MagaTailor]

Leading to this:

rustc: arm-unknown-linux-gnueabihf/stage0/lib/rustlib/arm-unknown-linux-gnueabihf/lib/libstd
src/libstd/lib.rs:1:1: 1:1 error: can't find crate for `alloc_jemalloc` [E0463]

which probably means this line always executes adding the jemalloc dependency to std. It used to work as expected a few weeks ago.

Even though I don't need this option on i686, it reproduces over there as well (as expected).

cc @alexcrichton

[MagaTailor]

Thx @simartin