x86: Make the divisor in setting `non_temporal_threshold` cpu specific

x86: Make the divisor in setting `non_temporal_threshold` cpu specific

Different systems prefer a different divisors.

From benchmarks[1] so far the following divisors have been found:
    ICX     : 2
    SKX     : 2
    BWD     : 8

For Intel, we are generalizing that BWD and older prefers 8 as a
divisor, and SKL and newer prefers 2. This number can be further tuned
as benchmarks are run.

[1]: https://github.com/goldsteinn/memcpy-nt-benchmarks
Reviewed-by: DJ Delorie <dj@redhat.com>