[roll third_party/pigweed] pw_thread: correct option setters to return a reference

Corrects the backend specific thread Options setters to return a
reference instead of a copy.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46240

third_party/pigweed Rolled-Commits: cf184e87d5dd23e..46cf0f166640374
Roller-URL: https://ci.chromium.org/b/8846758461725119120
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8846758461725119120
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If65d56564116ab41b8659538a0c6f25cfe527754
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/46261
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 cf184e8..46cf0f1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit cf184e87d5dd23e091ecc5f5046775def15ee530
+Subproject commit 46cf0f166640374186f629f9d2ca161650aa5ac0