meson: fix cpu_family for aarch64

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.