kernel: Move arch into kernel crate Change-Id: I2abb1c0c001597b4328e4fb995abb0f5a06aff00 Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/maize/+/262773 Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com> Reviewed-by: Travis Geiselbrecht <travisg@google.com> Commit-Queue: Erik Gilling <konkers@google.com>