Align on purego build tag.

[mattyclarkson]

golang/go#23172 proposes the de-facto purego build tag.

circl uses both purego and noasm build tags, inconsistently.

If a user wants a purego build, the build should not include assembly.

Would circl be willing to align on purego meaning noasm?

For example, this is what the upstream golang crypto package does: https://go-review.googlesource.com/c/go/+/561935

Another option is to make purego and noasm equivalent in the code-base if noasm needs to be a backwards supported build flag.

I would be willing to contribute the patch.

[armfazh]

@mattyclarkson thanks for raising this issue, happy to review your patch.