[roll third_party/pigweed] pw_system: Make I/O a facade

Makes pw_system's io library a facade to allow targets to use things
other than pw_sys_io as their transport.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/75940

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 2c042f9c0098c30..66c509979444b2d
Roller-URL: https://ci.chromium.org/b/8825848918992791137
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8825848918992791137
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3c679f8cb4c0eb2aba421d6a913858da9ad166df
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/76880
Bot-Commit: 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 2c042f9..66c5099 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 2c042f9c0098c30d132f0edf046dfa560bd18186
+Subproject commit 66c509979444b2de1954a12b9a8988066f5c65af