infra: fix simd_type detection for aarch64 cpus

'host_machine.cpu()' returns specific cpu model
(for example 'cortex-a72'), which would never match the condition.
This replaces it with the 'cpu_family()', as is used
in the conditions above, to match aarch64.

Note: 'cpu_family' values defined here:
https://mesonbuild.com/Reference-tables.html#cpu-families