roll: third_party/pigweed: pw_toolchain: Make toolchain registration configurable

Makes WORKSPACE toolchain registration configurable so projects can
manually control all the toolchains they want to register. This is
intended to help smooth over the migration to rules_cc toolchains.

Original-Bug: b/346388161
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/235712
Docs-Not-Needed: Armando Montanez <amontanez@google.com>

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 08ff555993b8ab2..40f756e2d3c40ee
Roller-URL: https://ci.chromium.org/b/8736879144270755585
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id464f839d3c47a34e76204846aa04027faf6f892
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/235892
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 08ff555..40f756e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 08ff555993b8ab250ea03a9f12aaf5c2d1c9c705
+Subproject commit 40f756e2d3c40eeb32832309dbcae989fb750268