As documented, it should be `aarch64` for AArch64.
* https://mesonbuild.com/Reference-tables.html#cpu-families
```
$ nix eval '((import <nixpkgs> {}).pkgsCross.aarch64-multiplatform.stdenv.targetPlatform.parsed.cpu.family)'
"arm"
```
The lookup table will ensure that, at any point, meson does not pick the
wrong family.