Emacs ports: Add lexical-binding cookie to site-wide initialization

As of 9f25d46568b in upstream's master branch, Emacs emits a run-time
warning when lexical binding is unset. This serves as a reminder that
lexical binding is preferred.

Reviewed by: yasu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49991