CMake: Check that backend targets are defined

Create a generator expression that fails to evaluate if a backend does
not exist. This avoids confusing compilation errors.

Change-Id: I5475fa53bf604b6cc445d9774b96fd8d5eb66f09
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44564
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Vic Yang <victoryang@google.com>