This change imports the AVX512 GC scanning kernel from CL 593938 into a
new package, internal/runtime/gc/scan. Credit to Austin Clements for
most of this work. I did some cleanup, added support for more size
classes to the expanders, and added more testing. I also restructured
the code to make it easier and clearer to add new scan kernels for new
architectures.
For #73581.
Change-Id: I76bcbc889fa6cad73ba0084620fae084a5912e6b
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64_avx512,gotip-linux-amd64_avx512-greenteagc
Reviewed-on: https://go-review.googlesource.com/c/go/+/655280
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
