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

This inadvertently changed to `[object Object]` with the V8 upgrade
in 8a24728a...96933df2. Use `Symbol.toStringTag` to undo this
particular change.

Fixes: https://github.com/nodejs/node/issues/9274
PR-URL: https://github.com/nodejs/node/pull/9279


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>