src: update postmortem constants

Update two postmortem constants in the ustack helper that
changed while moving to V8 7.0.

PR-URL: https://github.com/nodejs/node/pull/22754


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>