CMake: Revamp CMake build

- Define CMake cache variables (similar to gn args) for facade backends.
- Create simple initial toolchain files for GCC and Clang.
- Misc updates to pigweed.cmake and pw_build/CMakeLists.txt.
- Add or expand various CMakeLists.txt files.
- Watch CMake files in watch.py to support using CMake in pw watch.

Change-Id: I490d157485d02811a11f6a3589a0508ea1851f58
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22840
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_status/CMakeLists.txt b/pw_status/CMakeLists.txt
index 132c41e..3c5a0f0 100644
--- a/pw_status/CMakeLists.txt
+++ b/pw_status/CMakeLists.txt
@@ -12,4 +12,6 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+include($ENV{PW_ROOT}/pw_build/pigweed.cmake)
+
 pw_auto_add_simple_module(pw_status)