[third_party/pigweed] Roll 2 commits

87ce3c98de71f1d pw_string: Docs tweaks
1863631ae76c219 Bazel: stm32cube support

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: a74535c7d8b644d..87ce3c98de71f1d
Roller-URL: https://ci.chromium.org/b/8766037595949621089
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3f6793ba233abc382d967243adf6718399d69d45
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/177926
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index a74535c..87ce3c9 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a74535c7d8b644d3c36f9d0891dbc59c47e4c3ef
+Subproject commit 87ce3c98de71f1d2f6a29eb048c08800985d361d