Feature name for ARM NEON

[gnzlbg]

@parched mentioned here that clang and gcc both name the target feature to enable NEON on ARM processors just "simd".

This must be resolved before stabilization of the ARM SIMD Intrinsics.

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@alexcrichton could you add an ARM badge and a "blocking stabilization" badge to this issue? (and probably to the run-time feature detection issue for ARM as well?). We should prune all of them before writing the RFC for each parts of the library.

[alexcrichton]

It's ok, I'm sure before stabilization we'll do a pass through the issue tracker anyway.

[workingjubilee]

All the documentation I can find for Arm SIMD calls it Neon now. 🤷

e.g. Introducing Neon for Armv8-A