roll: third_party/pigweed 7940ef2..8706efb (5 commits)

8706efb:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/258392 pw_protobuf: Allow unset callback fields in message structs
bfe2c77:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256693 docs: Move bug directive
dbf07bb:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/258393 pw_toolchain: Add manual tag to supports_pic_old
924b629:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256835 fuchsia_sdk: Use toolchain defined in rules_fuchsia
e5e913d:https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256373 [owners] Remove luluwang@google.com from pw_bluetooth_proxy/OWNERS

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 7940ef25e097da..8706efb9847543
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8726463417189134433
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iddf06c8fbea388d64d0cc2a1603f0b461f957865
Reviewed-on: https://pigweed-review.googlesource.com/c/gonk/+/258396
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 7940ef2..8706efb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 7940ef25e097dae964b4e07004ad70942a9b0473
+Subproject commit 8706efb9847543d226c5cb17494faffcd034cfa0