add structure for normal SKX opts

My experience porting the old opts over to the current SKX opts setup
was so bad that I don't want to try any more, and think it's probably
safer to port the SKX code to the old setup.

Need to hook up dependents (Chromium and Google3 I think) before we can
move the actual SXK opts code over.

Change-Id: Ibb8bc4a083cb104cd39f27cbfbc16e9eedd9bd46
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294495
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed