lib: make String(global) === '[object global]'

This inadvertently changed to `[object Object]` with the V8 upgrade
in 8a24728...96933df. Use `Symbol.toStringTag` to undo this
particular change.

Fixes: nodejs#9274

Author: addaleax

lib/internal/bootstrap_node.js

@@ -188,6 +188,12 @@
   }
 
   function setupGlobalVariables() {
+    Object.defineProperty(global, Symbol.toStringTag, {
+      value: 'global',
+      writable: false,
+      enumerable: false,
+      configurable: true
+    });
     global.process = process;
     const util = NativeModule.require('util');
 

test/parallel/test-global.js

@@ -21,3 +21,5 @@ const fooBar = module.fooBar;
 assert.strictEqual('foo', fooBar.foo, 'x -> global.x in sub level not working');
 
 assert.strictEqual('bar', fooBar.bar, 'global.x -> x in sub level not working');
+
+assert.strictEqual(Object.prototype.toString.call(global), '[object global]');