[roll third_party/pigweed] pw_cpu_exception_cortex_m: use target for private constants

Sets up a shared build target for the private constants header
instead of using it as a source for multiple build targets.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56031

third_party/pigweed Rolled-Commits: 2253040d12b3133..6ef8a1d50a0ab2d
Roller-URL: https://ci.chromium.org/b/8839800068558291424
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839800068558291424
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If1e65cffe15e8a9f9e496669913bc42a3a9e4abd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56123
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 2253040..6ef8a1d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 2253040d12b3133e374d1814673f82d6f09f7565
+Subproject commit 6ef8a1d50a0ab2d97fafcb51d49906b813d59ec1