roll: third_party/pigweed pw_allocator: Add MeasureFragmentation

This CL provides an implementation of the algorithm to calculate a
fragmentation metric that was only previously described. This typically
shouldn't be invoked on device unless there is robust floating point
support. Adding this method allows platforms that do have floating
support to avoid implementing it themselves, as well as providing it for
host tasks like benchmarking.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/238417
Original-Revision: 020780642847dba69a9b2025f1f698fe3d8e4801

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 39f64a722bd247..020780642847db
Roller-URL: https://ci.chromium.org/b/8735248605418763249
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3576244c5167473470bf4441187c7cb76eaa0335
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/238870
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 39f64a7..0207806 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 39f64a722bd24730d936d9360f232a90d699f8a3
+Subproject commit 020780642847dba69a9b2025f1f698fe3d8e4801