move sk_memset?? to SkOpts

This lets the compiler generate AVX versions with wider writes.

Change-Id: Ia63825e70c72bdb4d14bef97d8b4ea4be54c9d84
Reviewed-on: https://skia-review.googlesource.com/17715
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed