tools: hide symbols for builtin JS files in binary

Do not expose symbols like `node::internal_process_next_tick_value`,
`node::internal_process_next_tick_key` in the created `node` binary
by wrapping them in an anonymous namespace.

PR-URL: #20634
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

tools/js2c.py

@@ -183,8 +183,12 @@ def ReadMacros(lines):
 
 namespace node {{
 
+namespace {{
+
 {definitions}
 
+}}  // anonymous namespace
+
 v8::Local<v8::String> LoadersBootstrapperSource(Environment* env) {{
   return internal_bootstrap_loaders_value.ToStringChecked(env->isolate());
 }}