[roll third_party/pigweed] pw_chrono: simplify RTOS SystemClock backends

Simplifies the SystemClock backends for FreeRTOS, ThreadX, and
embOS using the updated pw::sync::SpinLock facade which has a
constexpr constructor which removes the need for special early
boot handling.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/36581

third_party/pigweed Rolled-Commits: 41b2b3d7dbc2aa8..db455e64bb4a11a
Roller-URL: https://ci.chromium.org/b/8853127684482800608
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8853127684482800608
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I5049189c67b24481e6b35015a1682ccd942fffba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/36840
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 41b2b3d..db455e6 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 41b2b3d7dbc2aa812451ca210e927af9311c6277
+Subproject commit db455e64bb4a11ae8755beb82d4c9401dde99638