Add matrix multiplication benchmark

[ghost]

Hi! This is my first PR, so I hope I'm doing it correctly.

I started adding SIMD benchmarks from this WASM SIMD repo.

The first is a double precision matrix multiplication benchmark, which covers these previously not benchmarked instructions:

f64x2.add
f64x2.mul
v128.load64_splat

I also have a tiny bash script for listing which instructions are covered in the benchmarks, let me know if I should include that in the PR as well.

[zherczeg]

NIce! Would be good to split it into two benchmarks, because this way the performance of simd an non-simd variant cannot be compared

[ghost]

Hmm, then the result will need to be checked differently. I'll try using a CRC32 checksum, since that's quick to calculate and doesn't add much code.

[zherczeg]

A simple sum could be enough

[ghost]

That should do it.