multimedia/makemkv: Add support for aarch64

Upstream started to provide the (closed-source) makemkvcon binary for
armv6/armv7 and aarch64 as well. For aarch64, we have the required
linux-c7 userland, so let's enable this architecture.

Differential Revision: https://reviews.freebsd.org/D35757

(cherry picked from commit b6dbfbcad02d129721037a005420e0bef8df2885)