detect arm64 correctly in cmake

[machinaut]

Starting to getting this to work on my M1 MacBook, first thing is this detection misfires for me.

I'm seeing CMAKE_SYSTEM_PROCESSOR == arm64, and this fixes detecting that.

Still a ways away from supported, though. Here's the output of list_cpu_features:

arch            : aarch64
implementer     :   0 (0x00)
variant         :   0 (0x00)
part            :   0 (0x00)
revision        :   0 (0x00)
flags           :

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[machinaut]

@googlebot I signed it!

[machinaut]

Oh woah this is a way more complete version of what I was gonna do #150