Add support for WebAssembly SIMD128

[TerrorJack]

The WebAssembly spec already supports 128-bit fixed-width SIMD opcodes, see the spec and the clang builtins for details.

I'm wondering if the simdutf maintainers are interested in accepting a port to WebAssembly SIMD128. This won't be worked on immediately (for Safari it's only in TP and not release yet), but having a tracking ticket is good anyway, and I'm curious to hear what you folks think :)

[lemire]

Not only would it be welcome, but we would be happy to help.

[easyaspi314]

WASM is a blatant copy of very similar to NEON (just without half vectors), so it would probably be an easy port — especially since the emscripten toolchain includes a polyfill arm_neon.h from SIMDe.

I know that for xxHash, the arm_neon.h header had solid performance with literally no changes, so it could probably get away with only a few specializations. (e.g. alternative paths when half vectors are used)