[roll third_party/pigweed] pw_span: Include system span header only if fresh enough

Before this CL polyfill span was always included and relied on the
__cpp_lib_span define. Some older standard libraries could have this one
not defined which leads to ambiguity during compilation.
This CL fetches __cpp_lib_span version first and includes system span
header only if version is good enough, otherwise it fall backs to custom
implementation.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99540

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 39336b04b150402..a1858e3730f30bf
Roller-URL: https://ci.chromium.org/b/8810531180055733169
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8810531180055733169
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0e82a1e97add506730fbf8d6e6414517fb148d69
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/99528
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 39336b0..a1858e3 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 39336b04b15040237cc298da9756a16945054d57
+Subproject commit a1858e3730f30bfc4734d3a790d2057d534efff6