Use C++20 for C++ code

Previously, the default language version was being used and there were
issues linking to the STL. Make the language version explicit and
resolve the linking issue.

Change-Id: I97c383c11a3b58b8b0912a184ac942f15761fde9
Reviewed-on: https://pigweed-review.googlesource.com/c/open-dice/+/40020
Reviewed-by: Darren Krahn <dkrahn@google.com>
Commit-Queue: Andrew Scull <ascull@google.com>
diff --git a/toolchains/BUILD.gn b/toolchains/BUILD.gn
index 6f74757..8287c00 100644
--- a/toolchains/BUILD.gn
+++ b/toolchains/BUILD.gn
@@ -31,6 +31,9 @@
     "-std=c99",
     "-pedantic",
   ]
+
+  # Modern C++ for test and tooling.
+  cflags_cc = [ "-std=c++20" ]
 }
 
 config("enable_sanitizers") {