runtime: use scan kernels in scanSpan [green tea]

runtime: use scan kernels in scanSpan [green tea]

This is an extra 15-20% faster over the current sparse span scanning
when AVX512+GFNI is available and there's sufficient density.

For #73581.

Change-Id: I9688e09885dd76c5ccab7c492c85a7e14e18ee04
Reviewed-on: https://go-review.googlesource.com/c/go/+/665495
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>