[roll third_party/pigweed] pw_cpu_exception: Prepare for entry link deps migration

Adds a noop "pw_cpu_exception:entry_impl" target to enable
customers to migrate by adding this to their pw_build_LINK_DEPS
before the entry facade/backend is migrated to actually use this.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74343

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 648b210c92e6267..793018105f28351
Roller-URL: https://ci.chromium.org/b/8827759994130173713
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827759994130173713
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic87f410202cd933291446f97800a04e8957ad0b3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/74460
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 648b210..7930181 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 648b210c92e62673262c9a028bbb16d0694925e1
+Subproject commit 793018105f28351857d8443403fea22ebcb419ae