[roll third_party/pigweed] pw_log_sink: Switch backing sink to multisink

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38401

third_party/pigweed Rolled-Commits: 4cdc3e757014174..7a3c6525a6f231b
Roller-URL: https://ci.chromium.org/b/8851822022248624528
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8851822022248624528
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1adbf445711ae1b4aa952342c3aef028e9974aa5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/39264
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 4cdc3e7..7a3c652 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4cdc3e7570141740cc99499603d687a01a5db05d
+Subproject commit 7a3c6525a6f231b1984816cd1b37af58b2552424