[roll third_party/pigweed] pw_malloc_freelist: Remove dependency on pw_boot_armv7m

By moving the pointers to the heap low and high addresses to the
pw_malloc_Init() call, the hard dependency on the ARMv7M pw_boot
implementation can be removed.

Bugs: pwbug/432
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/54241

third_party/pigweed Rolled-Commits: 5bd954b0dc9f6d2..516255cae7c1806
Roller-URL: https://ci.chromium.org/b/8840968970642067808
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8840968970642067808
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8dad24fccfeee868736b42574ef5992aceb505ba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/54301
Reviewed-by: 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 5bd954b..516255c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5bd954b0dc9f6d2496e5cf570afd5965fa55d402
+Subproject commit 516255cae7c1806388ad3503d850c58240a6dc1e