Allow disabling jemalloc as the memory allocator

Requiring jemalloc as the allocator for all platforms hamstrings rust to being as portable as jemalloc is, with no exceptions. It is often more difficult to get jemalloc working in various situations, such as working with emscripten. With this motivation, a new configure option, `--disable-jemalloc` is provided to build liballoc to use the system allocator by default rather than jemalloc. The default is still to enable jemalloc.