[roll third_party/pigweed] pw_toolchain: Simplify macOS -nostdlib++ usage

Merges the nostdlib flag with the libc++.a link flag to make the intent
clearer.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/192898

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8a74b5a34b6efc5..6495a6ee1295c2f
Roller-URL: https://ci.chromium.org/b/8755900909092075393
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iea10eeafd1bf84c06b9cf5901fae3478a3d04e4f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/192937
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 8a74b5a..6495a6e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8a74b5a34b6efc56887be65cf35f08ee6ca73c1c
+Subproject commit 6495a6ee1295c2f6e042083cde9b7adef68108b6