portably determine HW SIMD width

[Bruce Carneal]

On Friday, 28 August 2020 at 00:19:12 UTC, kinke wrote:
> If x86 only, the predefined versions might do it - 
> `version(D_AVX2)` for 512 bit vectors (`-mattr=+avx2`), and 
> D_AVX for 256 bit. There aren't other alternatives at the 
> moment AFAIK; a trait might be a nice addition.

Thanks.  I was hoping to avoid the x86 tie-in but it's certainly 
better than nothing.