fix: package <__assertion_handler> as part of libcxx headers (#41830)

Refs https://chromium-review.googlesource.com/c/chromium/src/+/5208502

BUILD.gn

@@ -1466,8 +1466,10 @@ dist_zip("hunspell_dictionaries_zip") {
 }
 
 copy("libcxx_headers") {
-  sources = libcxx_headers + libcxx_licenses +
-            [ "//buildtools/third_party/libc++/__config_site" ]
+  sources = libcxx_headers + libcxx_licenses + [
+              "//buildtools/third_party/libc++/__assertion_handler",
+              "//buildtools/third_party/libc++/__config_site",
+            ]
   outputs = [ "$target_gen_dir/electron_libcxx_include/{{source_root_relative_dir}}/{{source_file_part}}" ]
 }