bazel: Mark toolchains_llvm as dev_dependency in MODULE.bazel

Mark toolchains_llvm bazel_dep and llvm extension usage as dev_dependency = True
so downstream consumers can provide their own LLVM or Pigweed toolchain
configurations without module resolution conflicts.

Change-Id: I46a5045d224889a99f292154b0b4fc5ea24e1c63
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/zephyrproject-rtos/zephyr/+/462336