[roll third_party/pigweed] pw_{chrono,sync,thread}: Group common backends in the GN build

Provide backend group variables for FreeRTOS and STL backends.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/215336
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: c7be3fb2f3c38dd..17c1ee08b087155
Roller-URL: https://ci.chromium.org/b/8745186926759102225
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3c9ab21f24f4b24e2814a8410c74629e721056d5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/216100
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index c7be3fb..17c1ee0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit c7be3fb2f3c38ddace41e66173e4fab2168f6661
+Subproject commit 17c1ee08b0871552ee50dd66ce2abd2c755e108a