[roll third_party/pigweed] pw_multibuf: Add SimpleAllocator

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/195129

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8f232f68faa1d72..38da87de52333c3
Roller-URL: https://ci.chromium.org/b/8753991085615606513
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3a9ab06d99a701caefc08dc58337abcf0674a4b4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/195512
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.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 8f232f6..38da87d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8f232f68faa1d7230ccb0f0d7c8840d9b946f941
+Subproject commit 38da87de52333c33d08f1e2ced4aac654e022cf6