4f5e73be1b
This is a hack to support building targets that don't support jemalloc alongside hosts that do. The jemalloc build is controlled by a feature of the std crate, and if that feature changes between targets, it invalidates the fingerprint of std's build script (this is a cargo bug); so we must ensure that the feature set used by std is the same across all targets, which means we have to build the alloc_jemalloc crate for targets like emscripten, even if we don't use it.