build: treat aarch64 as arm64

`Makefile` is now consistent with `configure` regarding
how we treat aarch64.

PR-URL: https://github.com/nodejs/node/pull/5191
Fixes: https://github.com/nodejs/node/issues/5175


Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>