src: update postmortem constant

Update a postmortem constant used by the ustack helper which
changed while moving to V8 7.1.

PR-URL: https://github.com/nodejs/node/pull/23423


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Myles Borins <myles.borins@gmail.com>