[Packaging] Add mimalloc to Linux builds

As a result of ARROW-11009/ARROW-11049/etc. it is now possible to switch allocators at runtime.  Previously only one allocator was compiled into the released builds (e.g. the PYPI version of pyarrow only included jemalloc on Linux and mimalloc on Windows).  It might now make sense to compile both mimalloc and jemalloc on Linux builds to allow the user to switch out to the appropriate allocator at runtime if they need to.