Split out memset opts into separate cpp files

These used to be in SkUtils_opts. I also renamed everything so that
they're declared in SkMemset.h, and defined in SkMemset_opts.h.

Bug: b/40045064
Bug: b/40045066
Change-Id: Idefa5b8d5f59595ddce907742f50e02174e42964
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/740638
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
41 files changed