Support float16 on ARM CPUs with native float16 support

[FlippFuzz]

From https://github.com/guillaumekln/faster-whisper/issues/65

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Some CPUs such as ARM Neoverse-N1 (Oracle Cloud free tier) support FP16 computation. It would be nice to have this feature because there could be up to a 2x speedup in computation speed compared to float32.

I'm just creating the enhancement request and understand that there might not be a focus on this because it only applies to a small subset of CPUs.