Remove simd on aarch64, but compile.

[jsoo1]

We compile programs that depend on hw-json for both x86_64 and aarch64. Of course the simd extensions won't be available on aarch64 but it would be nice if hw-json could compile for the platform, anyways.

I think this would also need to be done in the cursor libraries, too. Or maybe this should instead be done in hw-simd or hw-json-simd instead? What do you think?

[newhoggy]

Thanks for the PR. Are you satisfied with the undefined in the PR? If so, I'm happy to merge and release.

Also, it probably should use error "not implemented" rather than undefined.

[jsoo1]

I would be happy to change to error with a better explanation. Though I don't love user error prefix that would come with it.

[newhoggy]

undefined calls error internally anyway.