[roll third_party/pigweed] pw_libcxx: Only enable in clang builds

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/211941
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: ffaaf974d53a74a..69fa240add6d3f3
Roller-URL: https://ci.chromium.org/b/8746638310558566817
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I27fc38f0cfae2a036ff06ff2329460066e1cbfea
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/211944
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index ffaaf97..69fa240 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ffaaf974d53a74a62cfdd3ad2ed9b2b695e6d795
+Subproject commit 69fa240add6d3f3620d1cd05fd3b527111d46cd6