[SkContexts] Prefactor SkCPU Code

This CL prefactors (pre-refactors) the SkCPU code to make way for the new SkContext work. All of the changes remove SKCPU concepts, but the follow up CLs will add back similar concepts.

Change-Id: Iec003f5b1573bee967672f5a77196f01ffb6aff3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1326156
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
39 files changed