Use more standard AVX512 and AVX2 flags

This should make us more consistent with Chromium's compilation
and benchmarking. See https://crrev.com/c/7841852

I did an analysis of the generated SkRasterpipeline changes
and they are nearly identical [1]

[1] http://gpaste/4815535381544960

Change-Id: Ib88674187e120b54c2ed8075b27c2a7e5e7f7fc9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1232976
Reviewed-by: Thomas Smith <thomsmit@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>