process: fix process.exitCode handling for fatalException

* set process.exitCode before calling 'exit' handlers so that there will
  not be a situation where process.exitCode !== code in 'exit' callback
  during uncaughtException handling
* don't ignore process.exitCode set in 'exit' callback when failed with
  uncaughtException and there is no uncaughtException listener

PR-URL: https://github.com/nodejs/node/pull/21739


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>