bazel: Register C++ toolchains for root only

Bug: 362827235
Change-Id: I735eaa58021d825930a1e5bbc4aa05841d2bbaba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/238318
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Docs-Not-Needed: Ted Pudlik <tpudlik@google.com>

MODULE.bazel

@@ -160,6 +160,7 @@ register_toolchains(
     "//pw_toolchain/arm_gcc:arm_gcc_cc_toolchain_cortex-m33+nofp",
     "//pw_toolchain/host_clang:host_cc_toolchain_linux",
     "//pw_toolchain/host_clang:host_cc_toolchain_macos",
+    dev_dependency = True,
 )
 
 register_toolchains(