Support 128-bit SIMD on AArch64 via LLVM backend

Currently, SIMD primitives are only available on x86(_64). It would be nice to extend SIMD support to AArch64.

The initial step would be supporting 128-bit width SIMD with ASIMD (NEON) via LLVM backend. Support via NCG or consideration of SVE can follow.

I opened a MR at !10456 (closed).

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