Migrates all CMake facades to use BACKEND variables which are
defined in backend.cmake variables, similar to GN.
Change-Id: I63dd1a81aa0a796366f6c78c4823e6081a6463f1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/117413
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
