moves pw_trace-related build args into imports so toolchains can import
them and silence unused build-arg warnings for downstream projects that
use Pigweed's default host toolchain but don't use pw_trace.
Change-Id: I25cd8b59e2b05e0345e9ed738c4b06beff9a5069
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/36080
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
