Implement mmap support in library_lz4.js?

As noted in https://github.com/emscripten-core/emscripten/issues/9085#issuecomment-1226676158, it seems that lz4 does not support `mmap`, and thus `-s LZ4=1` can't be used for packaging things like message catalogs (as gettext ultimately relies on `mmap` for loading message catalogs).

@kripken, @sbc100: Would adding `mmap` support to `library_lz4.js` be practical / possible / in scope in the future?