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_build/BUILD.gn b/pw_build/BUILD.gn
index 6044286..d30bbc9 100644
--- a/pw_build/BUILD.gn
+++ b/pw_build/BUILD.gn
@@ -16,6 +16,9 @@
 
 import("$dir_pw_docgen/docs.gni")
 
+# IMPORTANT: The compilation flags in this file must be kept in sync with
+#            the CMake flags pw_build/CMakeLists.txt.
+
 config("colorize_output") {
   cflags = [
     # Colorize output. Ninja's Clang invocation disables color by default.