timers: make setImmediate() immune to tampering

Make setImmediate() immune to `process` global tampering by removing
the dependency on the `process._immediateCallback` property.

PR-URL: https://github.com/nodejs/node/pull/17736
Fixes: https://github.com/nodejs/node/issues/17681


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>