[gn build] (manually) port 3b62506 (-std=c++20) more

8c54583 ported this only for libcxx, not libcxxabi.
As of 05337a7, it's needed for libcxxabi too.

llvm/utils/gn/secondary/libcxxabi/src/BUILD.gn

@@ -65,7 +65,10 @@ config("cxxabi_config") {
     # Some files depend on libc++ internals.
     "//libcxx/src",
   ]
-  cflags_cc = [ "-nostdinc++" ]
+  cflags_cc = [
+    "-std=c++20",
+    "-nostdinc++",
+  ]
   defines = [ "_LIBCXXABI_BUILDING_LIBRARY" ]
   if (target_os == "win") {
     defines += [ "_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS" ]